Output signal processing circuit for eddy current sensor and output signal processing method for eddy current sensor

ABSTRACT

An eddy current sensor assembly includes an eddy current sensor and an output signal processing circuit that processes an output signal from the eddy current sensor. The output signal processing circuit includes a mixer circuit that accepts the output signal and a signal of the predetermined frequency as input, multiplies the two signals received as input, and outputs an output signal obtained by the multiplication, and a low-pass filter that accepts the output signal output by the mixer circuit as input, cuts a high-frequency signal included in the output signal received as input, and outputs at least a direct-current (DC) signal.

TECHNICAL FIELD

The present invention relates to a circuit and a method for processingan output signal from an eddy current sensor.

BACKGROUND ART

Eddy current, sensors are used for applications such as Mm thicknessmeasurement and displacement measurement. Hereinafter, film thicknessmeasurement will be taken as an example to describe an eddy currentsensor. An eddy current sensor for film thickness measurement is used ina semiconductor device fabrication step (polishing step), for example.In the polishing step, the eddy current sensor is used as follows. Asthe level of integration in semiconductor devices advances, circuitinterconnects are becoming finer, and the spacing between interconnectsis becoming narrower. Accordingly, it is necessary to planarize thesurface of a semi conductor wafer, which is a polishing target. It istherefore a common practice to polish the wafer by a polishing apparatusas a way of planarization.

A polishing apparatus is provided with a polishing table for holding apolishing pad for polishing the polishing target and a top ring forholding and pressing the polishing target against the polishing pad. Thepolishing table and the top ring are each rotated by a driving unit (amotor, for example). A liquid containing: a polishing agent (slurry) ismade to flow onto the polishing pad, and by pressing the polishingtarget held by the top ring against the polishing pad, the polishingtarget is polished.

If the polishing of the polishing target by the polishing apparatus isinsufficient, circuits will not be insulated front each other, possiblyleading to a short circuit. On the other hand, other problems may occurif the polishing target is over-polished, such as a rise in resistancevalues because of a decrease in the cross-sectional area of the wiring,or the wiring itself may be completely removed and the circuit itselfmay not be formed. Consequently, there is a demand to detect an optimalpolishing endpoint in a polishing apparatus.

Such technology is described in Japanese Patent Laid-Open No.2011-23579. In this technology, an eddy current sensor using three coilsis used to detect a polishing endpoint. As illustrated in FIG. 5 ofJapanese Patent Laid-Open No. 2011-23579, among the three coils, adetecting coil and a dummy coil form a series circuit, both ends ofwhich are connected to a resistance bridge circuit including a variableresistance. By adjusting the balance with the resistance bridge circuit,it is possible to adjust the zero point such that, the output of theresistance bridge circuit goes to zero when the film thickness is zero.The output of the resistance bridge circuit is input into a synchronousdetector circuit, as illustrated in FIG. 6 of Japanese Patent Laid-OpenNo. 2011-23579. The synchronous detector circuit extracts a resistancecomponent. (R), a reactance component (X), an amplitude output (Z), anda phase output (tan⁻¹R/X) associated with changes in the film thicknessfrom the input signal. With regard to a detection method using a bridgecircuit of the related art, the magnitude of adjustment to theresistance value when adjusting the zero point is extremely small,compared to the magnitude of the entire resistance value forming thebridge circuit. As a result, the magnitude of a temperature change inthe entire resistance value is non-negligible compared to the magnitudeof the resistance value adjustment when adjusting the zero point.Because of change, in the resistance value due to temperature change,change in the stray capacitance of the resistance, and the tike, theproperties of the bridge circuit are highly susceptible to the influenceof changes in the surrounding resistance environment. As a result, thereis a problem in that the zero point described above shifts easily, andthe accuracy of measuring the film thickness fails.

Also, with regard to the method of measuring the film thickness using aneddy current sensor of the related art, the signal output by the eddycurrent sensor is noisy. For this reason, when detecting tinyinterconnects (Cu interconnects, for example), the output signal itselfis small, and in some cases the output signal is buried in the noise andthe film thickness cannot be measured. Additionally, in the cases wherethe magnitude of the output signal is somewhat larger than, the noise,the film thickness is measurable, but there is still the problem of lowaccuracy of the film thickness measurement.

CITATION LIST Patent Literature

-   -   PTL 1: Japanese Patent Laid-Open No. 2011-23579

SUMMARY OF THE INVENTION Technical Problem

One aspect of the present invention has been devised in order to addresssuch issues, and an object thereof is to provide a circuit and a methodfor processing an output signal from an eddy current sensor that is lesssusceptible to the influence of changes in the surrounding environmentcompared to the related art.

Another aspect of the present invention has been devised in order toaddress other issues, and an object thereof is to provide a circuit anda method for processing an output signal from an eddy current sensor inwhich the signal-to-noise ratio (S/N) is improved over the relatedtechnology.

Solution to Problem

To address the above problems, Aspect 1 adopts a configuration of aneddy-current sensor assembly comprising: an eddy current sensor; and anoutput signal processing circuit, that processes an output signal of theeddy current sensor, wherein the output signal processing circuitincludes a mixer circuit that accepts the output signal and a signal ofa predetermined frequency as input, multiplies the two input signals,and outputs an output signal obtained by the multiplication, and alow-pass filter that accepts the output signal output by the mixercircuit as input, cuts a high-frequency signal included in the outputsignal received as input, and outputs at least a direct-current (DC)signal.

Aspect 2 adopts a configuration of an eddy current sensor output signalprocessing circuit that processes first and second output signals outputby an eddy current sensor including first and second coils thatrespectively output the first and second output signals, the outputsignal processing circuit comprising: a first mixer circuit that acceptsthe first, output signal and a signal of a predetermined frequency asinput, multiplies the two input signals, and outputs an output signalobtained by the multiplication; a first low-pass filter that accepts theoutput signal output by the first mixer circuit as input, cuts ahigh-frequency signal included in the output signal received as input,and outputs at least a DC signal; a second mixer circuit that acceptsthe second output signal and the signal of the predetermined frequencyas input, multiplies the two input signals, and outputs an output signalobtained by the multiplication; a second low-pass filter that acceptsthe output signal output by the second mixer circuit as input, cuts ahigh-frequency signal included in the output signal received as input,and outputs at least a DC signal; and a first subtractor circuit thataccepts the DC signal output by the first low-pass filter and the DCsignal output by the second low-pass filter as input, obtains adifference between the two input DC signals, and outputs the obtaineddifference.

Aspect 3 adopts a configuration of the eddy current sensor output signalprocessing circuit according to Aspect 2, wherein the output signalprocessing circuit includes a first adjustment circuit that accepts theDC signal output by the first low-pass filter as input, adjusts amagnitude of an amplitude of the input DC signal, and outputs anadjusted DC signal, and the first subtractor circuit accepts the DCsignal output by the first adjustment circuit and the DC signal outputby the second low-pass filter as input, obtains a difference between thetwo input DC signals, and outputs the obtained difference.

Aspect 4 adopts a configuration of the eddy current sensor output signalprocessing circuit according to Aspect 2, wherein the output signalprocessing circuit, includes a first adjustment-circuit that accepts theDC signal output by the first low-pass filter as input, adjusts amagnitude of an amplitude of the input DC signal, and outputs anadjusted. DC signal, and a second adjustment circuit that accepts the DCsignal output by the second low-pass filter as input, adjusts amagnitude of an amplitude of the input DC signal, and outputs anadjusted DC signal, and the first subtractor circuit accepts the DCsignal output by the first adjustment circuit and the DC signal outputby the second adjustment circuit as input, obtains a difference betweenthe two input. DC signals, and outputs the obtained difference.

Aspect 5 adopts a configuration of the eddy current sensor output signalprocessing circuit according to any one of Aspects 2 to 4, wherein theeddy current sensor includes third and fourth coils that respectivelyoutput third and fourth output signals, and the output signal processingcircuit includes a third mixer circuit that accepts the third outputsignal and the signal of the predetermined frequency as input,multiplies the two input signals, and outputs an output signal obtainedby the multiplication, a third low-pass filter that accepts the outputsignal output by the third mixer circuit as input, cuts a high-frequencysignal included in the output signal received as input, and outputs atleast a DC signal a fourth mixer circuit that accepts the fourth outputsignal and the signal of the predetermined, frequency as input,multiplies the two input signals, and outputs an output signal obtainedby the multiplication, a fourth low-pass filter that, accepts theoutput, signal output, by the fourth mixer circuit as input, cuts ahigh-frequency signal included in the output signal received as input,and outputs at least a DC signal, a second subtracter circuit thataccepts the DC signal output by the third low-pass filter and the DCsignal output by the fourth low-pass filter as input, obtains adifference between the two input DC signals, and outputs the obtaineddifference, and an adder circuit that accepts the difference output bythe first subtracter circuit and the difference output by the secondsubtractor circuit as input, obtains a sum of the two input differencesor a difference between the two input differences, and outputs theobtained sum or difference.

Aspect 6 adopts a configuration of an eddy current sensor output signalprocessing method comprising: inputting an output, signal of an eddycurrent sensor and a signal of a predetermined frequency into a mixercircuit, multiplying the two input signals with the mixer circuit, andoutputting an output signal obtained by the multiplication, andinputting the output signal output by the mixer circuit into a low-passfilter, cutting a high-frequency signal, and outputting at least a DCsignal.

Aspect 7 adopts a configuration of an eddy current sensor output signalprocessing method that, processes first and second output signals outputby an eddy current sensor including first and second coils thrurespectively output first and second output signals, the output signalprocessing method comprising: inputting the first output signal and asignal of a predetermined frequency into a first mixer circuit,multiplying the two input signals with the first mixer circuit andoutputting an output signal obtained by the multiplication; inputtingthe output signal output by the first mixer circuit into a firstlow-pass filter, cutting a high-frequency signal included in the outputsignal with the first low-pass filter, and outputting at least a DCsignal; inputting the second output signal and the signal of thepredetermined-frequency into a second mixer circuit, multiplying the twoinput signals with the second mixer circuit, and outputting an outputsignal obtained by the multiplication; inputting the output signaloutput by the second mixer circuit into a second low-pass filter,cutting a high-frequency signal included in the output signal with thesecond low-pass filter, and outputting at least a DC signal; andinputting the DC signal output by the first low-pass filter and the DCsignal output by the second low-pass filter into a first subtractorcircuit, obtaining a difference between the two input DC signals withthe first subtractor circuit, and outputting the obtained difference.

Aspect 8 adopts a configuration of an eddy current sensor output signalprocessing method wherein the eddy current sensor includes third andfourth coils that respectively output third and fourth output signals,and the output signal processing method comprises; inputting the thirdoutput signal and the signal of the predetermined frequency into a thirdmixer circuit, multiplying the two input signals with the third mixercircuit, and outputting an output signal obtained by the multiplication;inputting the output signal of the third mixer circuit into a thirdlow-pass filter, cutting a high-frequency signal included in the outputsignal with the third low-pass filter, and outputting at least a DCsignal; inputting the fourth output signal and the signal of thepredetermined frequency into a fourth mixer circuit, multiplying the twoinput signals with the fourth mixer circuit, and outputting an outputsignal obtained by the multiplication; inputting the output signal ofthe fourth mixer circuit into a fourth, low-pass filter, cutting ahigh-frequency signal included in the output signal with the fourthlow-pass filter, and outputting at least a DC signal; inputting the DCsignal output by the third low-pass filter and the DC signal output bythe fourth low-pass filter into a second subtractor circuit, obtaining adifference between the two input DC signals with the second subtractorcircuit, and outputting the obtained difference; and inputting thedifference output by the first subtracter circuit and the differenceoutput by the second subtractor circuit into an adder circuit, obtaininga sum of the two differences or a difference between the two differenceswith the adder circuit, and outputting the obtained sum or difference.

Aspect 9 adopts a configuration of a polishing apparatus comprising: apolishing table to which a polishing pad is attached for polishing asubstrate; a driving unit configured to rotationally drive the polishingtable; a holding unit configured to hold the substrate and press thesubstrate against the polishing pad; the eddy current sensor disposedinside the polishing table and configured to detect an eddy currentformed in a conductive film formed on the substrate in association withthe rotation of the polishing table; the eddy current sensor outputsignal processing circuit according to any one of Aspects 2 to 5; and anendpoint detection controller configured to compute film thickness dataabout the substrate from the output of the output signal processingcircuit.

To address the above problems, Aspect 10 adopts a configuration of aneddy current sensor assembly comprising: an eddy current sensorincluding an exciting coil capable of accepting an excitation, signaland generating a magnetic field, and a detecting coil capable ofdetecting the magnetic field and outputting a detection signal; and anoutput signal processing circuit that processes the detection signal,wherein the output signal processing circuit includes a generatorcircuit capable of generating a noise reduction signal for reducingnoise from the excitation signal or the detection signal, and an addercircuit capable of adding the noise reduction signal generated in thegenerator circuit to the detection signal to generate a noise-reducedsignal in which noise included in the detection signal is reduced.

Aspect 11 adopts a configuration of an eddy current sensor output signalprocessing circuit that processes a first detection signal and a firstdummy signal output by as eddy current sensor including a first excitingcoil capable of accepting an excitation signal as input and generating afirst magnetic field, a first detecting coil capable of detecting thefirst magnetic field and outputting the first detection signal, and afirst dummy coil capable of detecting the first magnetic field andoutputting the first dummy signal, the output signal processing circuitcomprising: a first resistance bridge circuit capable of outputting adifference between the first detection, signal and the first dummysignal as a first difference signal; a first generator circuit capableof generating a first noise reduction signal for reducing noise from anyof the excitation signal, the first detection signal, the first dummysignal and the first difference signal; and a first adder circuitcapable of adding the first noise reduction signal generated in thefirst generator circuit to the first difference signal to generate afirst noise-reduced signal in which noise included in the firstdifference signal is reduced.

Aspect 12 adopts a configuration of the eddy current sensor outputsignal processing circuit according to Aspect 11, wherein the eddycurrent sensor includes a second exerting coil capable of accepting theexcitation signal as input and generating a second magnetic field, asecond detecting coil capable of detecting the first magnetic field andthe second magnetic field, and outputting a second defection signal, anda second dummy coil capable of detecting the first magnetic field andthe second magnetic field, and outputting a second dummy signal, thefirst detecting coil is capable of detecting the first magnetic fieldand the second magnetic field to output the first detection signal thefirst dummy coil is capable of detecting the first magnetic field andthe second magnetic field to output the first dummy signal, and theoutput signal processing circuit, includes a second resistance bridgecircuit capable of outputting a difference between the second detectionsignal and the second dummy signal as a second difference signal, asecond generator circuit capable of generating a second noise reductionsignal for reducing noise from any of the excitation signal, the seconddetection signal the second dummy signal, and the second differencesignal, a second adder circuit capable of adding the second noisereduction signal generated in the second generator circuit to the seconddifference signal to generate a second noise-reduced signal in whichnoise included, in the second difference signal, is reduced, and a thirdadder circuit capable of adding together the first noise-reduced signaland the second noise-reduced signal.

Aspect 13 adopts a configuration of an eddy current, sensor outputsignal processing circuit that processes first and second detectionsignals and first and second dummy signals output by an eddy currentsensor including first and second exciting coils capable of accepting anexcitation signal as input and respectively generating first and secondmagnetic fields, first and second detecting coils capable of detectingthe first magnetic field and the second magnetic field, and respectivelyoutputting the first and second detection signals, and first and seconddummy coils capable of detecting the first magnetic field and the secondmagnetic field, and respectively outputting the first and second dummysignals, the output signal processing circuit comprising: a firstresistance bridge circuit capable of outputting a difference between thefirst detection signal and the first dummy signal, as a first differencesignal; a second resistance bridge circuit capable of outputting adifference between the second detection signal and the second dummysignal as a second difference signal, a third adder circuit capable ofadding together the first difference signal and the second differencesignal.

Aspect 14 adopts a configuration of an eddy current sensor output sigoal processing method that processes a detection signal output by aneddy current sensor including an exciting coil capable of accepting anexcitation signal as input and generating a magnetic field, and adetecting coil capable of detecting the magnetic field and generatingthe detection signal, the output signal processing method comprising:generating a noise reduction signal for reducing noise from theexcitation signal or the detection signal; and adding the generatednoise reduction signal to the detection signal to generate anoise-reduced signal in which noise included, in the detection signal isreduced.

Aspect 15 adopts a configuration of an eddy current sensor output signalprocessing method that processes a first detection signal and a firstdummy signal output by an eddy current sensor including a first excitingcoil capable of accepting an excitation signal as input and generating afirst magnetic field, a first detecting coil capable of detecting thefirst magnetic field and outputting the first detection, signal, and afirst dummy coil capable of detecting the first magnetic field andoutputting the first dummy signal, the output signal processing methodcomprising: outputting a difference between the first detection, signaland the first dummy signal as a first difference signal; generating afirst noise reduction signal for reducing noise from any of theexcitation signal, the first detection signal, the first dummy signal,and the first difference signal; and adding the generated first noisereduction signal to the first difference signal to generate a firstnoise-reduced signal in which noise included in the first differencesignal is reduced.

Aspect 16 adopts a configuration of the eddy current sensor outputsignal processing method according to Aspect 15, wherein the eddycurrent sensor includes a second exciting coil capable of accepting theexcitation signal as input and generating a second magnetic field, asecond detecting coil capable of detecting the first magnetic field andthe second magnetic Held, and outputting a second detection signal, anda second dummy coil capable of detecting the first magnetic field andthe second magnetic field, and outputting a second dummy signal, thefirst detecting coil is capable of detecting the first magnetic fieldand the second magnetic field to output the first detection signal, thefirst dummy coil is capable of detecting the first magnetic field andthe second magnetic field to output the first dummy signal, and theoutput signal processing method comprises: outputting a differencebetween the second detection signal and the second dummy signal as asecond difference signal; generating a second noise reduction signal forreducing noise from any of the excitation signal, the second detectionsignal, the second dummy signal, and the second difference signal;adding the generated second noise reduction signal to the seconddifference signal to generate a second noise-reduced signal in whichnoise included in the second difference signal is reduced; and addingtogether the first noise-reduced signal and the second noise-reducedsignal.

Aspect 17 adopts a configuration of an eddy current sensor output signalprocessing method that processes first and second detection signals andfirst and second dummy signals output by an eddy current sensorincluding first and second exciting coils capable of accepting anexcitation, signal as input and respectively generating first and secondmagnetic fields, first and second detecting coils capable of detectingthe first magnetic field and the second magnetic field, and respectivelyoutputting the first and second detection, signals, and first and seconddummy coils capable of detecting the first magnetic field and the secondmagnetic field, and respectively outputting the first and second dummysignals, the output signal processing method comprising: outputting adifference between the first detection signal and the first dummy signalas a first difference signal; outputting a difference between the seconddetection signal and the second dummy signal as a second differencesignal; adding together the first difference signal and the seconddifference signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an overall configuration of apolishing apparatus according to an embodiment of the present invention;

FIG. 2 is a plan view illustrating the relationship among a polishingtable, an eddy current sensor, and a semiconductor wafer;

FIG. 3A is a block diagram illustrating a configuration of an eddycurrent sensor assembly;

FIG. 3B is an equivalent circuit diagram of the eddy current, sensorassembly;

FIG. 4 is a block diagram illustrating the eddy current, sensor assemblyof the present embodiment;

FIG. 5 is a schematic diagram illustrating an exemplary configuration ofa coil in the eddy current sensor of the present embodiment;

FIG. 6 is a schematic diagram illustrating an output signal processingcircuit according to another embodiment;

FIG. 7 is a diagram illustrating a resistance bridge circuit;

FIG. 8 is a schematic diagram illustrating an exemplary configuration ofan eddy current sensor of another embodiment;

FIG. 9 is a schematic diagram illustrating aa exemplary connection of anexciting coil in the eddy current sensor;

FIG. 10 is a diagram illustrating a magnetic field generated by the eddycurrent sensor;

FIG. 11 is a schematic diagram illustrating an output signal processingcircuit according to another embodiment;

FIG. 12 is a perspective view of the eddy current sensor according tothe other embodiment illustrated in FIG. 8 ;

FIG. 13 is a schematic diagram illustrating a circuit configurationinside a digital signal processor;

FIG. 14A is a diagram illustrating how a direct-current (DC) signalchanges depending on a phase difference between an output signal of theeddy current sensor and a signal from an alternating-current (AC) signalsource:

FIG. 14B is a diagram illustrating how the DC signal changes dependingon the phase difference between the output signal of the eddy currentsensor and the signal from the AC signal source;

FIG. 14C is a diagram illustrating how the DC signal changes dependingon the phase difference between the output signal of the eddy currentsensor and the signal from the AC signal source;

FIG. 14D is a diagram illustrating how the DC signal changes dependingon the phase difference between the output signal of the eddy currentsensor and the signal from the AC signal source;

FIG. 15 is a schematic diagram illustrating an overall configuration ofa polishing apparatus according to another embodiment of the presentinvention;

FIG. 16 is a block diagram illustrating an eddy current sensor assemblyof the present embodiment;

FIG. 17 is a block diagram illustrating the basic principles of noisecanceling according to the present embodiment;

FIG. 18 is a block diagram illustrating a synchronous detector circuitof the eddy current sensor;

FIG. 19A is a schematic diagram illustrating an exemplary connection, ofeach coil in the eddy current sensor:

FIG. 19B is a schematic diagram illustrating an exemplary connection ofeach coil in the eddy current sensor;

FIG. 19C is a schematic diagram illustrating an exemplary connection ofeach coil in the eddy current sensor;

FIG. 20 is a perspective view of the eddy current sensor illustrated inFIG. 8 and a block diagram illustrating an exemplary connection;

FIG. 21 is a perspective view of the eddy current sensor illustrated inFIG. 8 and a block diagram illustrating an exemplary connection;

FIG. 22 is a spectrum illustrating an intensity distribution of noiseand signal contained in an output signal from a detecting coil withrespect to frequency; and

FIG. 23 is a spectrum illustrating the intensity distribution of noiseand signal contained in the output signal from the detecting coil withrespect to frequency.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings. Note that in the following embodiments,identical or corresponding members are denoted with the same signs, anda duplicate description may be omitted. Also, features indicated in eachembodiment are also applicable to other embodiments insofar as theembodiments do not contradict each other,

FIG. 1 is a schematic diagram illustrating an overall configuration of apolishing apparatus to which an eddy current sensor 50 according to anembodiment of the present invention is applied. As illustrated in FIG. 1, the polishing apparatus is provided with a polishing table 100 and atop ring (holding unit) 1 that holds a substrate to be polished, such asa semiconductor wafer, against a polishing surface on the polishingtable.

The polishing table 100 is coupled to a motor (not illustrated) thatacts as a driving unit disposed underneath through a table spindle 100a, and is capable of rotating about the table spindle 100 a. A polishingpad 101 is affixed to the top face of the polishing table 100, and thesurface 101 a of the polishing pad 101 forms a polishing surface thatpolishes a semiconductor wafer WH. A polishing liquid supply nozzle 102is installed above the polishing table 100, such that a polishing liquidQ is supplied onto the polishing pad 101 on the polishing table 100 bythe polishing liquid supply nozzle 102, As illustrated in FIG. 1 , aneddy current sensor 50 is embedded inside the polishing table 100.

The top ring 1 basically includes a top ring body 142 that presses thesemiconductor wafer WH against the surface 101 a and a retainer ring 143that holds the peripheral edges of the semiconductor wafer WH such, thatthe semiconductor wafer WH does not fly off the top sing.

The top ring 1 is connected to a top ring shaft 111, and the top ringshaft 111 is configured to move up and down with respect to a top ringhead 110 by a raising/lowering mechanism 124. By the up and downmovement of the top ring shaft 111, the entire top ring 1 is raised orlowered and positioned with respect to the top ring head 110. Note thata rotary joint 125 is attached to the top end of the top ring shaft 111.

The raising/lowering mechanism 124 that moves the top ring shaft 111 andthe top ring 1 up and down is provided with a bridge 128 that rotatablysupports the top ring shaft 111 through a bearing 126, a ball screw 132attached to the bridge 128, a support stand 129 supported by a supportcolumn 130, and a servo motor 138 provided on the support stand 129. Thesupport stand 129 that supports the servo motor 138 is secured to thetop ring head 110 through the support column 130.

The ball screw 132 is provided with a screw shaft 132 a coupled to theservo motor 138 and a nut 132 b with which the screw shaft 132 aengages. The top ring shaft 1H is configured to move up and down as onewith the bridge 128. Consequently, when the servo motor 138 is driven,the bridge 128 moves up and down through the hall screw 132, therebycausing the top ring shaft 111 and the top ring 1 to move up and down.

Additionally, the top ring shaft 111 is coupled to a rotating cylinder112 through a key (not illustrated). The rotating cylinder 112 isprovided with a timing pulley 113 on the outer periphery thereof. A topring motor 114 is secured to the top ring head 110, and the timingpulley 113 is connected to a timing pulley 116 provided in the top ringmotor 114 through the timing belt 115, Consequently, by rotationallydriving the top ring motor 114, the rotating cylinder 112 and the topring shaft 111 rotate as one through, the timing pulley 116, the timingbelt 115, and toe timing pulley 113, and the top ring 1 rotates. Motethat the top nog head 110 is supported by a top ring head shaft 117rotationally supported by a frame (not illustrated).

In the polishing apparatus configured as illustrated in FIG. 1 , the topring 1 is capable of holding a substrate such as the semiconductor waferWH on the bottom face thereof. The top ring head 110 is capable ofturning about the top ring head shaft 117, and the turning of the topring head 110 causes the top ring 1 holding the semiconductor wafer WHon the bottom face to move from a semiconductor wafer WH receivingposition to above the polishing table 100. Thereafter, the top ring 1 islowered to press the semiconductor wafer WH against the surface(polishing surface) 101 a of the polishing pad 101. At this time, thetop ring 1 and the polishing table 100 are each made to rotate, and thepolishing liquid Q is supplied onto the polishing pad 101 from thepolishing liquid supply nozzle 102 provided above the polishing table100, in this way, the surface of the semiconductor wafer WH is polishedby causing the semiconductor wafer WH to slide against the polishingsurface 101 a of the polishing pad 101,

FIG. 2 is a plan view illustrating the relationship among the polishingtable 100, the eddy current sensor 50, and the semiconductor wafer WH.As illustrated in FIG. 2 , the eddy current sensor 50 is installed at aposition that passes through the center Cw of the semiconductor wafer WHbeing polished that is held by the top ring 1. The polishing table 100rotates about a rotation center 160. For example, the eddy currentsensor 50 is capable of detecting a metal film (conductive film) such asa Cu layer of the semiconductor wafer WH continuously on a passagetrajectory (scan line) while passing under the semiconductor wafer WH.

Next, the eddy current sensor 50 provided in the polishing apparatusaccording to the present invention will be described in further detailusing the attached drawings.

FIGS. 3A and 3B are diagrams illustrating a configuration of an eddycurrent sensor assembly including the eddy current sensor 50, in whichFIG. 3A is a block diagram illustrating the configuration of the eddycurrent sensor assembly, and FIG. 3B is an equivalent circuit diagram,of the eddy current sensor assembly. As illustrated in FIG. 3A, the eddycurrent sensor 50 is disposed near a metal film (or conductive film) mfto be detected, and an alternating-current (AC) signal source 52 isconnected to a coil of the eddy current sensor 50. Here, the metal dim(or conductive film) mf to be detected is a thin film of a material suchas Cu, Al, Au, or W formed on the semiconductor wafer WH, for example.The eddy current sensor 50 is disposed near the metal film (orconductive film) to be detected at a distance of approximately 1.0 mm to4.0 mm for example. The coil is normally wound around a magneticmaterial such as ferrite (not illustrated). The eddy current sensor 50may also be an air-core coil.

The type of signal detection by the eddy current sensor is referred toas the impedance type, in which an eddy current is generated in themetal film (or conductive film) mf to cause a change in the impedance,and the metal film (or conductive film) is detected from the impedancechange. In other words, with, the impedance type, changing the eddycurrent I₂ causes the impedance Z to change in the equivalent circuitillustrated in FIG. 3B, and if the impedance Z as seen from the signalsource (a fixed frequency oscillator) 52 changes, the change in theimpedance Z can be detected by the output signal processing circuit 54,and a change in the metal film (or conductive film) can be detected.

In an eddy current sensor of the impedance type, it is possible toextract signal outputs X and Y, the phase, and the combined impedance Z(=X+iY). From the impedance components X, Y, and the like, measurementinformation about the thickness of the metal film (or conductive film)of Cu, Al, Au, or W is obtained. As illustrated in FIG. 3 , the eddycurrent sensor 50 can be built into the polishing table 100 at aposition near the surface and positioned to face the semiconductor waferto be polishing through the polishing pad, such that changes in themetal film (or conductive film) can be detected from an eddy currentflowing through the metal film (or conductive film) on the semiconductorwafer.

For the frequency of the eddy current sensor, a single radio wave,AM-modulated radio waves, the sweep output from a function generator, orthe like can be used, and it is preferable to select an oscillatingfrequency and a modulation method with good sensitivity to match thetype of metal film.

Hereinafter, an eddy current sensor of the impedance type will bedescribed specifically. The AC signal, source 52 is an oscillator of afixed frequency approximately from 2 MHz to 3.0 MHz, for which a quartzoscillator is used for example. Additionally, a current I₁ flows throughthe eddy current sensor 50 due to an AC voltage supplied by the ACsignal source 52. By causing a current to flow through the eddy currentsensor 50 positioned near the metal, film (or conductive film) mf, themagnetic flux links with the metal film (or conductive film) mf to forma mutual inductance M between the two, and an eddy current I₂ flowsthrough the metal film (or conductive film) mf Here, R1 is theequivalent resistance on the primary side that includes the eddy currentsensor, and L1 is the self-inductance on the primary side that similarlyincludes the eddy current sensor. On the metal film (or conductive film)mf side, R2 is the equivalent resistance corresponding to eddy currentloss, and L2 is the self-inductance thereof. The impedance 2 seen on theeddy current sensor side from terminals a and b of the AC signal source52 changes depending on the magnitude of the eddy current loss formed,in the metal film (or conductive film) mf.

FIG. 1 also illustrates the output signal processing circuit 54 of theeddy current sensor 50. As illustrated in FIG. 2 , the polishing table100 of the polishing apparatus is capable of rotating about an axis 170,as indicated by the arrow. The AC signal source 52 and the output signalprocessing circuit 54 are embedded inside the polishing table 100. Theeddy current sensor 50 may also be integrated with the AC signal source52 and the output signal processing circuit 54. An output signal 172from the output signal processing circuit 54 enters the table spindle100 a of the polishing table 100 and passes through a rotary joint (notillustrated) provided on the axial end of the table spindle 100 a ₅thereby connecting the output signal processing circuit 54 to anendpoint detection controller 246 by the output signal 172. Note that atleast one of the AC signal source 52 and the output signal processingcircuit 54 may also be disposed outside the polishing table 100.

FIG. 4 illustrates an eddy current sensor assembly 174, The eddy currentsensor assembly 174 includes the eddy current sensor 50 and the outputsignal processing circuit 54 that processes an output signal 176 fromthe eddy current sensor 50. The output signal processing circuit 54accepts the output Signal 176 and a signal 180 of the predeterminedfrequency as input. The mixer circuit 178 multiplies the two inputsignals (the output signal 176 and the signal 180), and outputs anoutput signal 182 obtained by the multiplication. The output signalprocessing circuit 54 additionally includes a low-pass filter 184. Thelow-pass filter 184 accepts the output signal 182 output by the mixercircuit 178 as input, cuts a high-frequency signal included in theoutput signal 182 received as input, and outputs an output signal 186including at least a DC signal.

The eddy current sensor 50 includes an exciting coil for forming an eddycurrent in the metal film (or conductive film) on the semiconductorwafer WH, and a detecting coil that detects the generated eddy current.For example, the exciting coil and the detecting coil are disposed inthe axial direction of a cylindrical ferrite core. The exciting coil isconnected to the AC signal source 52, The exciting coil forms an eddycurrent in the metal film (or conductive film) mf on the semiconductorwafer WH disposed near the eddy current sensor 50 due to a magneticfield formed by the voltage supplied by the AC signal source 52, Thedetecting coil is disposed on the upper side (the metal flint (orconductive film) side) of the ferrite core, and detects the magneticfield generated by the eddy current formed in the metal film (orconductive film). In FIG. 4 , the exciting coil is not illustrated. Thesignal 180 having the same frequency as the voltage supplied to theexciting coil is supplied to the mixer circuit 178 from the AC signalsource 52.

The mixer circuit 178 is an analog multiplier or a digital multiplier.The mixer circuit 178 is a circuit that accepts two voltage signalshaving the same (or different) frequency components f1 and f2, andperforms a multiplication operation, on both signals. The mixer circuit178 is also referred to by terms such as mixer, mixing circuit,frequency mixer, frequency mixing circuit, MIX, frequency transducer,frequency conversion circuit, and frequency converter. The purpose ofthe mixer circuit is to multiply two input, signals and extract a sum ora difference of respective frequencies of the two signals (performfrequency conversion), in the case where the respective frequencies ofthe two input signals are the same, the phase difference between, thetwo signals can be extracted.

The mixer circuit 178 includes an input port 198, a local oscillatorport 200, and an output port 202, The output signal 176 from the eddycurrent sensor 50 is input into the input port 198. The signal 180 fromthe AC signal source 52 is input into the local oscillator port 200. Theoutput signal 176 and the signal 180 have the same frequency. The resultof multiplying the output signal 176 and the signal 180 in the mixercircuit 178 is output from the output port 202, The output signal 182output by the output port 202 is input into the low-pass filter 184. Bypassing through the low-pass filter 184, the amount of phase change(phase difference) between the output signal 176 and the signal 180detected by the eddy current sensor 50 is extracted as a DC signal (theoutput signal 186). The cutoff frequency of the low-pass filter can beset in the range from 1 Hz to 1.0 Hz, for example.

The processing performed by the mixer circuit 178 is as follows. Let theoutput signal 176 be A·sin(ωt+θ_(A)). Let the signal 180 beB·sin(ωt+θ_(B)). In the above, A and B are the respective amplitudes(mv, for example) of the output signal 176 and the signal 180, ω is thefrequency (1/s), t is the time (s), and θ_(A) and θ_(B) are therespective phases of the output signal 176 and the signal 180. Theoutput signal 182 from the mixer circuit 178 isA·sin(ωt+θ_(A))·B·sin(ωt+θ_(B))=(½)A·B·cos(ωt+θ_(A)+ωt+θ_(B))+(½)A·B·cos(ωt+θ_(A)−ωt−θ_(B))(½)A·B·cos(2ωt+θ_(A)+θ_(B))+(½)A·B·cos(θ_(A)−θ_(B)). When this signal isinput into the low-pass filter 184, the first term above is eliminated,and the output signal 186 from the low-pass filter 184 is the secondterm (½)·A·B·cos(θ_(A)−θ_(B)) above, or in other words, a DC signal.

In the present embodiment, the output signal 176 from the eddy currentsensor 50 is connected to the mixer circuit 178 without going through aresistance bridge circuit. Because a resistance bridge circuitsusceptible to the influence of temperature changes is not used, it ispossible to provide the output signal processing circuit 54 of the eddycurrent sensor 50 that is less susceptible to the influence of changesin the surrounding environment compared to the related art.

Also, as illustrated in FIG. 6 of Japanese Patent Laid-Open No.2011-23579, a signal detected at the terminal of a resistance bridgecircuit passes through a high-frequency amplifier and a phase shiftcircuit, and the cos component and the sin component of the detectionsignal are extracted by a synchronous detection unit containing a cossynchronous detector circuit and a sin synchronous detector circuit. Inthe synchronously detected signals, an unwanted high-frequency componentabove the signal component is removed by a low-pass filter,Consequently, the high-frequency output that the coil outputs becomes alow-frequency signal after passing through long processing steps.Because there are many steps involving a high-frequency signal thesignal is noisy in the related art.

In the present embodiment, the high-frequency output (output signal 176)output by the cod of the eddy current sensor 50 becomes a low-frequencysignal (output signal 186) by the mixer circuit 178 and the low-passfilter 184. Because there are few steps involving the output signal 176which is a high-frequency signal the output Signal 186 contains lessnoise compared to the related art. In other words, because the signal isconverted from a high-frequency signal susceptible to the influence ofthe surroundings into an easy-to-handle DC signal at an early stage (infew steps), consistent signal measurement can be performed.

In FIG. 4 , when the signal 180 and the output signal 176 are compared,in the case where the phase change between the two signals is large andthe amplitude change is small, it is preferable to measure the filmthickness according to the phase change. In the case where the phasechange between the two signals is small and the amplitude change issmall, it is preferable to measure the film thickness according to theamplitude change. In the case where the phase change between the twosignals is large and the amplitude change is also large. It ispreferable to measure the film thickness according to the phase changeand the amplitude change. Additionally, the signal 180 does not have tobe a sine wave. For example, a square wave of the same frequency as theoutput signal 176 may also be used. Furthermore, it is sufficient forthe frequencies of the output signal 176 and the signal 180 to besubstantially the same without being strictly the same. In the case of alarge frequency discrepancy between the output signal 176 and the signal180, at least one of the frequencies of the output signal 176 and thesignal 180 may be input into the mixer circuit 178 after being adjustedby a frequency adjustment circuit.

Next, a different embodiment of the present invention wilt be described.FIG. 5 is a schematic diagram illustrating an exemplary configuration ofa coil in the eddy current sensor 50 of the present embodiment. In thepresent embodiment, the eddy current sensor 50 includes an exciting coil72 for forming an eddy current in the metal film (or conductive film), adetecting coil 73 for detecting the eddy current of the metal film (orconductive film), and a dummy coil 74. The eddy current sensor 50includes the coils of the exciting coil 72, the detecting coil 73, andthe dummy coil 74 in three layers wound around a ferrite core 71, Notethat the structure of the eddy current sensor 50 is not limited to thestructures illustrated in FIGS. 4,5, and 8 , and any structure may beadopted.

Here, the exciting coil 72 in the center is connected to the AC signalsource 52. The exciting coil 72 forms an eddy current in the metal film(or conductive Him) mf on the semiconductor wafer WH disposed near theeddy current sensor 50 due to a magnetic field formed by the voltagesupplied by the AC signal source 52. The detecting coil 73 is disposedon the upper side (the metal film (or conductive film) side) of theferrite core 71, and detects the magnetic field generated by the eddycurrent formed in the metal film (or conductive film). Additionally, thedummy coil 74 is disposed on the side of the exciting coil 72 oppositethe detecting coil 73, The exciting coil 72, the detecting coif 73, andthe dummy coil 74 are coils having the same number of turns (from 1 t to20 t), for example. The reason for providing the dummy coil 74 is toenable the output from the output signal processing circuit 54 to beadjusted to zero when a metal film (or conductive film) is not present.

FIG. 6 is a schematic diagram illustrating the output signal processingcircuit 54 according to the present embodiment. The eddy current sensor50 includes the detecting coil 73 (first coil) and the dummy cod 74(second coil) that output the output signal 176 (first output signal)and an output signal 188 (second output signal), respectively. Theoutput signal processing circuit 54 processes the output signal 176 andthe output signal 188 output by the eddy current sensor 50. The outputsignal processing circuit 54 includes a mixer circuit 1781 (first mixercircuit) that accepts the output signal 176 and the signal 180 of thepredetermined frequency output by the AC signal source 52 as input,multiplies the output signal 176 and the signal 180 received as input,and outputs an output signal 182 obtained by the multiplication, and alow-pass filter 1841 (first low-pass filter) that accepts the outputsignal 182 output by the mixer circuit 1781 as input, cuts ahigh-frequency signal included in the output signal 182 received asinput, and outputs an output signal 1861 including at least a DC signal.

The output signal processing circuit 54 additionally includes a mixercircuit 1782 (second mixer circuit) that accepts the output signal 188and the signal 180 of the predetermined frequency output by the ACsignal source 52 as input, multiplies the output signal 188 and thesignal ISO received as input, and outputs an output signal 190 obtainedby the multiplication, and a low-pass filter 1842 (second low-passfilter) that accepts the output signal 190 output by the mixer circuit1782 as input, cuts a high-frequency signal included in the outputsignal 190 received as input, and outputs an output signal 1862including at least a DC signal.

The output signal processing circuit 54 additionally includes a firstsubtracter circuit 1961 (subtracter circuit) that accepts the outputsignal 1861 output by the low-pass filter 1841 and the output signal1862 output by the low-pass filter 1842 as input, calculates adifference between the two output signals 1861 and 1862 received asinput, and outputs the obtained difference.

Furthermore, the output signal processing circuit. 54 may also include afirst adjustment circuit 1941 that accepts the output signal 1861 (DCsignal) output by the low-pass filter 1841 as input, adjusts themagnitude of the amplitude of the output signal 1861 received as input,and outputs an adjusted DC signal 1921 and a second adjustment circuit1942 that accepts the output signal 1862 (DC signal) output by thelow-pass filter 1842 as input, adjusts the magnitude of the amplitude ofthe output signal 1862 received as input, and outputs an adjusted DCsignal 1922, and additionally include a first subtracter circuit 1961(subtracter circuit) that, calculates a difference between the adjustedDC signal 1921 output by the first adjustment circuit 1941 and theadjusted DC signal 1922 output by the second adjustment circuit 1942 asinput, and outputs the obtained difference.

Also in the present embodiment, the output signal 176 from the eddycurrent sensor 50 and the output signal IBS are connected to the misercircuit 1781 and the mixer circuit 1782 without going through aresistance bridge circuit. Because a resistance bridge circuitsusceptible to the influence of temperature changes is not used, it ispossible to provide the eddy current sensor output signal processingcircuit 54 that is less susceptible to the influence of changes in thesurrounding environment compared to the related art.

In the present embodiment, the first subtractor circuit 1961 is capableof performing detection similar to the detection method using a bridgecircuit of the related art, in which two coils are used to extract atiny signal change corresponding to a film thickness change. In otherwords, it is possible to calculate the difference between two DC signalsto detect only a tiny signal change corresponding to a film thicknesschange.

At this point. FIG. 7 will be used to describe a detection method usinga bridge circuit as a comparative example. As illustrated in FIG. 7 , ofthe three coils of the exciting coil 72, the detecting coil 73, and thedummy coil 74, the detecting coil 73 and the dummy coil 74 form a seriescircuit, both ends of which are connected to a resistance bridge circuit204 that includes a variable resistance 206, a variable resistance 208,and a fixed resistance 210. By adjusting the balance with the variableresistance 206 and the variable resistance 208 of the resistance bridgecircuit 204, it is possible to adjust the zero point such that theoutput of the resistance bridge circuit 204 goes to zero when the filmthickness is zero. The output of the resistance bridge circuit 204 isinput into a synchronous detector circuit 214, as illustrated in FIG. 7. The synchronous detector circuit 214 extracts a resistance component(R), a reactance component (X), an amplitude output (Z), and a phaseoutput (tan⁻¹R/X) associated with changes in the film thickness from theinput signal. A stray capacitance 212 of the resistance Is generated inthis circuit.

The magnitude of adjustment to the resistance values of the variableresistance 206 and the variable resistance 208, or in other words themagnitude of adjustment to the resistance value during zero-pointadjustment, is extremely small compared to the magnitude of the entireresistance value forming the bridge circuit. As a result, the magnitudeof a temperature change in the entire resistance value is non-negligiblecompared, to the magnitude of the zero-point adjustment. Because ofchange in the resistance value due to temperature change, the straycapacitance of the resistance, and the like, the properties of theresistance bridge circuit 204 are highly susceptible to the influence ofchanges in the surrounding resistance environment. As a result, there isa problem in that the zero point described above shifts easily, and theaccuracy of measuring the film thickness falls.

Returning to the description of FIG. 6 , the first subtractor circuit1961 accepts the DC signal 1921 output by the first adjustment circuit1941 and the DC signal 1922 output by the second adjustment circuit 1942as input, calculates a difference between the two DC signals 1921 and1922 received as input, and outputs the obtained difference. The reasonsfor providing the first adjustment circuit 1941 and the secondadjustment circuit 1942 are as follows.

With an adjustment circuit and a subtractor circuit, it is possible tomaintain substantially the same performance as the detection methodusing a resistance bridge circuit of the related art, in which two coils(the detecting coil 73 and the dummy coil 74) are used to extract a tinysignal change corresponding to a film thickness change. Namely, thelevel of the phase detection signal from at least one of the coils (butin the ease of FIG. 6 , the two coils of the detecting coil 73 and thedummy coil 74) is subtracted after being adjusted by the adjustmentcircuit to detect only a tiny signal change corresponding to a filmthickness change. With the adjustment circuit, the accuracy of thezero-point adjustment cart be improved, compared to the case without theadjustment circuit. Consequently, tiny changes in the signal from thezero point described above can be extracted. It is also acceptable toprovide only one of the first adjustment circuit 1941 and the secondadjustment circuit 1942, This is because even with a single adjustmentcircuit, zero-point adjustment is possible in some cases by adjustingthe level of the phase detection signal with the adjustment circuit.

The first adjustment circuit 1941 and the second adjustment circuit 1942are attenuators, for example. An attenuator refers to a circuit elementor device that attenuates a signal input into the attenuator to asuitable signal level (amplitude). The first adjustment circuit 1941 andthe second adjustment circuit 1942 may also be amplifiers, for example.Besides the reasons originating from the coil properties, the followingare also reasons for providing the first adjustment circuit 1941 and thesecond adjustment circuit 1942.

A mixer circuit or a low-pass filter may output an input signal with anamplified or attenuated amplitude in some cases. In such cases, thefirst adjustment circuit 1941 and the second adjustment circuit 1942 arenecessary to adjust the amplitude of the output from the mixer circuitor the low-pass circuit.

Next, a different embodiment of the present invention will be described.FIGS. 8 and 9 are schematic diagrams illustrating an exemplaryconfiguration of the eddy current sensor 50 and an exemplary connectionof the exciting coil in the eddy current sensor according to the presentembodiment. The eddy current sensor 50 disposed near the substrate onwhich a conductive film is formed includes a pot core 60 and six coils860, 862, 864, 866, 868, and 870. The pot core 60 is a magnetic materialand has a floor part 61 a (bottom magnetic material), a magnetic corepart 61 b (central magnetic material) provided in the center of thebottom face part 61 a, and a surrounding wall part 61 c (peripheralmagnetic material) provided at the periphery of the floor part 61 a. Thesurrounding wall part 61 c is a wall provided at the periphery of thefloor part 61 a so as to surround the magnetic core part 61 b. In thepresent embodiment, the floor part 61 a has a circular disc shape, themagnetic core part 61 b has a solid columnar shape, and the surroundingwall part 61 c has a cylindrical shape surrounding the floor part 61 a.

Of the six coils 860, 862, 864, 866, 868, and 870, the coils 860 and 862in the center are exciting coils connected to the AC signal source 52,The exciting coils 860 and 862 form an eddy current in the metal film(or conductive film) mf on the semiconductor wafer WH disposed nearbydue to a magnetic field formed by the voltage supplied by the AC signalsource 52. The detecting coils 864 and 866 are disposed on the metalfilm side of the exciting coils 860 and 862, and detect the magneticfield generated by the eddy current formed in the metal film. The dummycoils 868 and 870 are disposed on the opposite side from the detectingcoils 864 and 866 with the exciting coils 860 and 862 in between.

The exciting coil 860 is an inner coil disposed on the outer peripheryof the magnetic core part 61 b, is capable of generating a magneticHeld, and forms an eddy current in a conductive film. The exciting coil862 is an outer coil disposed on the outer periphery of the surroundingwall part 61 c, is capable of generating a magnetic Held, and forms aneddy current in a conductive film. The detecting coil 864 is disposed onthe outer periphery of the magnetic core part 61 b, is capable ofdetecting a magnetic field, and detects an eddy current formed in aconductive film. The detecting coil 866 is disposed on the outerperiphery of the surrounding wall part 61 c, is capable of detecting amagnetic field, and detects an eddy current formed in a conductive film.

The eddy current sensor includes the dummy coils 868 and 870 that detectan eddy current formed in a conductive film. The dummy coil 868 isdisposed on the outer periphery of the magnetic core part 61 b, and iscapable of detecting a magnetic field. The dummy coil 870 is disposed onthe outer periphery of the surrounding wall part 61 c, and is capable ofdetecting a magnetic field. In the present embodiment, the detectingcoils and the dummy coils are disposed on the outer periphery of thefloor part 61 a and the outer periphery of the surrounding wall part 61c, but the detecting coils and the dummy coils may also be disposed ononly one of either the outer periphery of the floor part 61 a or theouter periphery of the surrounding wall part 61 c.

The axial direction of the magnetic core part 61 b is orthogonal to theconductive film on the substrate, the detecting coils 864 and 866, theexciting coils 860 and 862, and the dummy coils 868 and 870 are disposedat different positions in the axial direction of the magnetic core part61 b, and additionally, the detecting coils 864 and 866, the excitingcoils 860 and 862, and the dummy coils 868 and 870 are disposed to theabove order in the axial direction of the magnetic core part. 61 bproceeding from a position close to the conductive film on the substrateto a position farther away, A lead line (illustrated in FIG. 11 ) forconnecting to the outside extends from each, of the detecting coils 864and 866, the exciting coils 860 and 862, and the dummy coils 868 and870.

FIG. 8 is a cross-section in a plane passing through a central axis 872of the magnetic core part 61 b. The pot core 60 is a magnetic materialand has a columnar floor part 61 a, a columnar magnetic core part 61 bpro vided in the center of the floor part 61 a, and a cylindricalsurrounding wall part 61 c provided on the circumference of the floorpart 61 a. As one example of the dimensions of the pot core 60, thediameter LE1 of the floor part 61 a is approximately from 1 cm to 5 cm,and the height, LE2 of the eddy current sensor 50 is approximately from1 cm to 5 cm. The outer diameter of the surrounding wall part 61 c hasthe same cylindrical shape in the height direction in FIG. 8 , but theouter diameter may also have a converging shape (tapered shape) thatnarrows in the direction going away from the floor part 61 a, or inother words, narrows toward the tip.

The conducting wire used in the detecting coils 864 and 866, theexciting coils 860 and 862, and the dummy coils 868 and 870 is copper,Manganin wire, or nichrome wire. As a result of using Manganin wire ornichrome wire, there are fewer temperature changes due to electricalresistance and the like, and the temperature properties are improved.

In the present embodiment, wire rod is wrapped around the outside of themagnetic core part 61 b and the outside of the surrounding wall part 61c made of ferrite or the like to form the exciting coils 860 and 862,and therefore the eddy current density flowing through the measurementtarget can be raised. Additionally, because the detecting coils 864 and866 are also formed on the outside of the magnetic core part 61 b andthe outside of the surrounding wall part 61 c, the generated reversemagnetic field (interlinkage flux) can be collective efficiently.

To raise the eddy current density flowing through the measurementtarget, in the present embodiment, the exciting coil 860 and theexciting coil 862 are additionally connected in parallel, as illustratedin FIG. 9 . In other words, the inner coil and the outer coil areelectrically connected in parallel. The reasons for connecting inparallel are as follows. When connected in parallel, the voltage thatcan be applied to the exciting coil 860 and the exciting coil 862 isincreased, and the current flowing through the exciting coil 860 and theexciting coil 862 is increased over the case of connecting in series.Consequently, the magnetic field is larger. Also, when connected inseries, the inductance of the circuit Increases, and the frequency ofthe circuit falls. This makes it difficult to apply tire necessary highfrequency to tire exciting coils 860 and 862. The arrow 874 indicatesthe direction of current flowing through the exciting coil 860 and theexciting coil 862.

As illustrated in FIG. 9 , the exciting coil 860 and the exciting coil862 are connected such that the exciting coil 860 and the exciting coil862 have the same magnetic field direction. In other words, currentflows in different directions in the exciting coil 860 and the excitingcoil 862. A magnetic field 876 is the magnetic field generated by theexciting coil 860 on the inner side, while a magnetic field 878 is themagnetic field generated by the exciting coil 862 on the outer side. Asillustrated in FIG. 10 , the magnetic field directions of the excitingcoil 860 and the exciting coil 862 are the same, in other words, thedirection of the magnetic field that the inner coil generates inside themagnetic core part 61 b is the same as the direction of the magneticfield that, the outer coil generates inside the magnetic core part 61 b.

Because the magnetic field 876 and the magnetic field 878 illustrated inthe region 880 have the same orientation, the two magnetic fields areadded together and become larger. Compared to the case where only themagnetic field 876 generated by the exciting coil 860 exists like in therelated art, in the present embodiment, the magnetic field is larger bythe extent of the magnetic field 878 generated by the exciting coil 862.

The detecting coil 864, the exciting coil 860, and the dummy coil 868correspond to the detecting coil 73, the exciting coil 72, and the dummycoil 74 of FIG. 5 . The detecting coil 866, the exciting coil 862, andthe dummy coil 870 correspond to the detecting coil 73, the excitingcoil 72, and the dummy coil 74 of FIG. 5 . In other words, the eddycurrent sensor of FIG. 10 has a structure in which two of the eddycurrent sensor in FIG. 5 are disposed concentrically. Accordingly, theoutput signal processing circuit 54 corresponding to the eddy currentsensor of FIG. 10 preferably includes two of the output signalprocessing circuit 54 illustrated in FIG. 6 .

An example of the output signal processing circuit 54 corresponding tothe eddy current sensor of FIG. 10 is illustrated in FIG. 11 . The eddycurrent sensor 50 includes the detecting coil 864 (first coil) and thedummy coil 868 (second coil) that output the output signal 176 (firstoutput signal) and the output signal 188 (second output signal),respectively. The eddy current sensor 50 includes the detecting coil 866(third coil) and the dummy coil 870 (fourth, coil) that output a thirdoutput signal 1761 and a fourth, output signal 1881, respectively.

The processing by the mixer circuit 1781, the mixer circuit 1782, thelow-pass filter 1841, and the low-pass filter 1842 associated with thefirst output signal and the second output signal is as described above,and therefore a description is omitted.

The output signal processing circuit 54 includes a third mixer circuit1783 that accepts the third output signal 1761 and the signal 180 of thepredetermined frequency output by the AC signal source 52 as input,multiplies the two signals received as input, and outputs an outputsignal 1821 obtained by the multiplication, and a third low-pass filter1843 that accepts the output signal 1821 output by the third mixercircuit 1783 as input, cuts a high-frequency signal included, in theoutput signal 1821 received as input, and outputs an output signal 1863including at least a DC signal.

The output signal processing circuit 54 includes a fourth mixer circuit1784 that accepts the fourth output signal 1881 and the signal 180 ofthe predetermined frequency output by the AC signal source 52 as input,multiplies the two signals received as input, and outputs an outputsignal 1901 obtained by the multiplication, and a fourth, low-passfilter 1844 that accepts the output signal 1901 output by the fourthmixer circuit 1784 as input, cuts a high-frequency signal included inthe output signal 1901 received as input, and outputs an output signal1864 including at least a DC signal.

An internal configuration of a digital signal processor 216 isillustrated in FIG. 13 . The output signal processing circuit 54includes a second subtractor circuit 1962 that accepts the output signal1863 output by the third, low-pass filter 1843 and the output signal.1864 output by the fourth low-pass filter 1844 as input, calculates adifference 2222 between the two DC signals received as input, andoutputs the obtained difference 2222, and an adder circuit 224 thatreceives the difference 2221 output by the first subtractor circuit 1961and the difference 2222 output by the second subtracter circuit 1962 asinput, calculates a sum of the two differences or a difference betweenthe two differences received as input, and outputs the obtained sum ordifference as the output signal 172.

In the present embodiment, the digital signal processor (DSP) 216includes the functions of the first, second, third, and fourthadjustment circuits, the first and second subtractor circuits, and theadder circuit. In other words, the digital signal processor 216 may alsoinclude a third adjustment circuit 1943 that receives the output signal1863 (DC signal) output by the third low-pass filter 1843 as input,adjusts the magnitude of the amplitude of the output signal 1863received as input, and outputs an adjusted DC signal 1923, and a fourthadjustment circuit 1944 that receives the output signal 1864 (DC signal)output by the fourth low-pass filter 1844 as input, adjusts themagnitude of the amplitude of the output signal 1864 received as Input,and outputs an adjusted DC signal 1924, The digital signal processor 216is a microprocessor suitable for digital signal processing. At least oneof the first, second, third, and fourth adjustment circuits, the firstand second subtractor circuits, and the adder circuit may also beprovided independently as an analog circuit or a digital circuit.

The present embodiment includes a coil assembly with two coils inaddition to the coil assembly with two coils illustrated in FIG. 5 . Inother words, there are two pairs of two coils. Because there are twopairs, the measurement accuracy is improved by obtaining the sum of thedifferences between the two DC signals or the difference of thedifferences between the two DC signals obtained by the two pairs. Themeasurement accuracy is improved because of the following reasons. Inthe case of taking the sum, the signal output is increased, and sincesmaller signal changes, or in other words smaller film thickness changescan be detected, the measurement accuracy is improved.

In the case of taking the difference, one pair is capable of measuringthe film thickness over a wider area than the other pair, for example.In this case, by subtracting the output of one pair from the other pair,the influence of the him thickness over a wide area can be reduced andonly the change in film thickness over a narrow area can be measuredmore accurately. In other words, the spatial precision of measurementcan be improved,

FIG. 12 illustrates a perspective view of the eddy current sensorillustrated in FIG. 8 , In FIG. 12 , a top face 218 is illustrated abovea top face 220 for easier understanding, but the top face 218 and thetop face 220 are in the same horizontal plane as illustrated in FIG. 8 .In FIG. 12 , there are two coil assemblies, but there may also be threeor more coil assemblies. In the case where there are two or more coilassemblies, the measurement accuracy (S/N ratio) is improved because ofthe increased number of film thickness measurements compared to the caseof a single coil assembly.

In the present embodiment, the output signal from the eddy currentsensor likewise is connected to the mixer circuit without going througha resistance bridge circuit. Because a resistance bridge circuitsusceptible to the influence of temperature changes is not used, it ispossible to provide the eddy current sensor output signal processingcircuit that is less susceptible to the influence of changes in thesurrounding environment compared to the related art.

In the present embodiment, the subtractor circuit is capable ofperforming detection similar to the detection method using abridgecircuit of the related art, in which two coils are used to extract atiny signal change corresponding to a film thickness change. In otherwords, it is possible to calculate the difference between two DC signalsto detect only a tiny signal change corresponding to a film thicknesschange.

The multiplier circuit and the subtractor circuit may also be a circuit,that outputs the result of the multiplication and subtraction multipliedby a predetermined constant (where the constant may be 1, less than 1,or greater 1), or in other words, outputs an amplified output signal.

FIGS. 14A, 14B, 14C, and 14D illustrate how the output signal 186, whichis a DC signal, changes depending on the phase difference between theoutput signal 176 from the eddy current sensor 50 (illustrated in FIG. 4) and the signal 180 from the AC signal source 52. FIGS. 14A, 14B, 14C,and 14D illustrate cases where the phase difference is 0 degrees, 30degrees, 60 degrees, and 90 degrees, respectively. As the phasedifference becomes larger, the output signal 186 becomes smaller. Is thegraphs, the horizontal axis is time (s) and the vertical, axis isamplitude (mv).

Next, a method of processing the output signal from the eddy currentsensor will be described, in FIG. 4 , the output signal 176 from theeddy current sensor 50 and the signal 180 from the AC signal source 32are input into the mixer circuit 178, The two signals are multiplied bythe mixer circuit 178, and the output signal 182 obtained by themultiplication is output. The output signal 182 output by the mixercircuit 178 is input into the low-pass filter 184, a high-frequencysignal is cut, and at least the DC signal 186 is output.

Next, a different method of processing the output signal from the eddycurrent sensor will be described. In FIG. 6 , the first and secondoutput signals 176 and 188 output by the eddy current sensor 50including the first and second coils 73 and 74 that respectively outputthe first and second output signals 176 and 188 are processed.

The first output signal 176 and the signal 180 from the AC signal source52 are input into the first mixer circuit 1781, the two signals aremultiplied by the first mixer circuit 1781, and the output signal 182obtained by the multiplication is output. The output signal 182 outputby the first mixer circuit 1781 is input into the first low-pass filter1841, a high-frequency signal included in the output signal 182 is cutby the Fust low-pass filter 1841, and at least the DC signal 1861 isoutput.

The second output signal 188 and the signal 180 from the AC signalsource 52 are input into the second mixer circuit 1782, the two signalsare multiplied by the second mixer circuit 1782, and the output signal190 obtained by the multiplication is output. The output signal 190output by the second mixer circuit 1782 is input into the secondlow-pass filter 1842, a high-frequency signal included in the outputsignal 190 is cut by the second low-pass filter 1842, and at least theDC signal 1862 is output.

The DC signal 1861 output by the first low-pass filter 1841 and the DCsignal 1862 output by the second low-pass filter 1842 are input into thefirst subtractor circuit 1961, the difference 172 between the two DCsignals is obtained by the first subtractor circuit 1961, and theobtained difference 172 is output.

Next, a different method of processing the output signal from the eddycurrent sensor will be described. In FIG. 11 , the eddy current sensorincludes the third and fourth coils 866 and 870 that output the thirdand fourth output signals 1761 and 1881, respectively. The third outputsignal 1761 and the signal 180 from the AC signal source 52 are input,into the third mixer circuit 1783, the two signals are multiplied by thethird mixer circuit 1783, and the output signal 1821 obtained by themultiplication is output.

The output signal 1821 from the third mixer circuit 1783 is input intothe third low-pass filter 1843, a high-frequency signal included in theoutput signal 1821 is cut by the third low-pass filter 1843, and atleast, the DC signal 1863 is output. The fourth output signal 1881 andthe signal 180 from the AC signal source 52 are input into the fourthmixer circuit 1784, the two signals are multiplied by the fourth mixercircuit 1784, and the output signal 1901 obtained by the multiplicationis output.

The output signal 1901 from the fourth mixer circuit 1784 is input intothe fourth low-pass filter 1844, a high-frequency signal included in theoutput signal 1901 is cut by the fourth low-pass filter 1844, and atleast the DC signal 1864 is output. The DC signal 1863 output by thethird low-pass filter 1843 and the DC signal 1864 output by the fourth,low-pass filter 1844 are input into the second subtractor circuit 1962,the difference 2222 between the two DC signals 1863 and 1864 is obtainedby the second subtracter circuit 1962, and the obtained difference 2222is output. The difference 2221 output by the first subtractor circuit1961 and the difference 2222 output by the second subtracter circuit1962 are input into the adder circuit 224. The sum of the twodifferences 2221 and 2222 or the difference between the two differences2221 and 2222 is obtained by the adder circuit 224, and the obtained sum172 or difference 172 is output.

A method of controlling each unit of the polishing apparatus on thebasis of the film thickness obtained by the eddy current sensor 50 isdescribed hereinafter. As illustrated in FIG. 1 , the eddy currentsensor 50 is connected to an endpoint detection controller 246, and theendpoint detection controller 246 is connected to an equipmentcontroller 248. The output signal from the eddy current sensor 50 issent to the endpoint detection controller 246, The endpoint detectioncontroller 246 performs necessary processing (arithmetic processing andcorrection) on the output signal from the eddy current sensor 50 togenerate a monitoring signal (film thickness data corrected by theendpoint detection controller 246). The equipment controller 248controls components such as the top ring motor 114 and a motor for thepolishing table 100 (not illustrated) on the basis of the corrected filmthickness data.

FIG. 15 is a schematic diagram illustrating an overall configuration ofa polishing apparatus to which an eddy current sensor 50 according toanother embodiment of the present invention is applied. As illustratedin FIG. 15 , the polishing apparatus is provided with a polishing table100 and a top ring (holding unit) 1 that holds a substrate to bepolished, such as a semiconductor wafer, against a polishing surface onthe polishing table.

The polishing table 100 is coupled to a motor (not illustrated) thatacts as a driving unit disposed underneath through a table spindle 170,and is capable of rotating about the table spindle 170, A polishing pad101 is affixed to the top face of the polishing table 100, and thesurface 101 a of the polishing pad 101 forms a polishing surface thatpolishes a semiconductor wafer WH. A polishing liquid supply nozzle 102is installed above the polishing table 100, such that a polishing liquidQ is supplied onto the polishing pad 101 on the polishing table 100 bythe polishing liquid supply nozzle 102. As illustrated in FIG. 15 , aneddy current sensor 50 is embedded inside the polishing table 100.

Hereinafter, an eddy current sensor of the impedance type according tothe present embodiment will be described specifically. The AC signalsource 52 includes an oscillator 260 (see FIG. 16 ) with a fixedfrequency approximately from 2 MHz to 30 MHz. The oscillator 260 is aquartz oscillator, for example. Additionally, a current I₁ flows throughthe eddy current sensor 50 due to an AC voltage supplied by the ACsignal source 52. By causing a current to flow through the eddy currentsensor 50 positioned near the metal film (or conductive film) mf, themagnetic flux links with the metal film (or conductive film) mf to forma mutual inductance M between the two, and an eddy current I₂ flowsthrough the metal film (or conductive film) mf. Here, R1 is theequivalent resistance on the primary-side that includes the eddy currentsensor, and L1 is the self-inductance on the primary side that similarlyincludes the eddy current sensor. On the metal film (or conductive film)mf side, R2 is the equivalent resistance corresponding to eddy currentloss, and L2 is the self-inductance thereof. The impedance Z seen on theeddy current sensor side from terminals a and b of the AC signal source52 changes depending on the magnitude of the eddy current loss formed inthe metal film (or conductive film) mf.

FIG. 15 also illustrates the output signal processing circuit 34 of theeddy current sensor 50. As illustrated in FIG. 2 , the polishing table100 of the polishing apparatus is capable of rotating about an axis 170,as indicated by the arrow. The AC signal source 52 and the output signalprocessing circuit 54 are embedded inside the polishing table 100. Theeddy current sensor 50 may also be integrated with the AC signal source52 and the output signal processing circuit 54. An output signal 172from the output signal processing circuit 54 enters the table spindle100 a of the polishing table 100 and passes through a rotary joint (notillustrated) provided on the axial end of the table spindle 100 a,thereby connecting the output signal processing circuit 54 to anendpoint detection controller 246 by the output signal 172. Note that atleast one of the AC signal source 52 and the output signal processingcircuit 54 may also be disposed outside the polishing table 100.

FIG. 16 illustrates an eddy current sensor assembly 174. The eddycurrent sensor assembly 174 includes the eddy current sensor 50 and theoutput signal processing circuit 54 that processes an output signal 176from the eddy current sensor 50. The eddy current sensor 50 includes anexciting coil 72 capable of accepting an excitation signal 250 as inputand generating a magnetic field 308 (see FIG. 19A), and a detecting coil73 capable of detecting the magnetic field 308 and outputting an output,signal 176 (detection signal). The output signal processing circuit. 54includes a generator circuit 254 capable of generating a noise reductionsignal 252 for reducing noise from the excitation, signal 250 or theoutput signal 176, and an adder circuit 258 capable of adding the noisereduction signal 252 generated in the generator circuit 254 to theoutput signal 176 and generating a noise-reduced signal 256 in which thenoise included in the output signal 176 is reduced. In the presentembodiment, the generator circuit 254 generates the noise reductionsignal 252 from the output signal 176 of the detecting coil 73. In thepresent embodiment, the output signal 176 is subjected to signalprocessing by a filter 266 and an amplifier 268, The generator circuit254 generates the noise reduction signal 252 from the processed outputsignal 176.

The AC signal source 52 may also include an amplifier 262 that amplifiesan output signal 270 from the oscillator 260, and a filter 264 forreducing noise included in an output signal 272 from the amplifier 262,Because noise is also amplified by the amplifier 262, the filter 264 isinstalled downstream of the amplifier 262 to reduce the amplified noise.In the present embodiment, the output signal from the filter 264 is theexcitation signal 250.

The eddy current sensor 50 includes an exciting coil 72 for forming aneddy current in the metal film (or conductive film) on the semiconductorwafer WH, and a detecting coil 73 that detects the generated eddycurrent. For example, the exciting coil 72 and the detecting coil 73 aredisposed in the axial direction of a cylindrical ferrite core. Theexciting coil 72 is connected to the AC signal source 52. The excitingcoil 72 forms an eddy current in the metal film (or conductive film) mfon the semiconductor wafer WH disposed near the eddy current sensor 50due to a magnetic field formed by the voltage supplied by the AC signalsource 52. The detecting coil 73 is disposed on the upper side (themetal film (or conductive film) side) of the ferrite core, and detectsthe magnetic field generated by the eddy current formed in the metalfilm (or conductive film). The eddy current sensor 50 may also include adummy coil 74 as described later.

The output signal 176 is input into the generator circuit 254 throughthe filter 266 and the amplifier 268 of the output signal processingcircuit 54 An amplifier not illustrated is disposed upstream of thefilter 266. The filter 266 is installed to reduce amplified noise. Inonly the amplifier not illustrated, the output signal 176 may be weak insome cases, and therefore the amplifier 268 is disposed downstream ofthe filter 266. An output signal 274 from the amplifier 268 is inputinto the generator circuit 254 and the adder circuit 258.

The generator circuit 254 generates the noise reduction signal 252. Thenoise reduction signal 252 is generated as follows. The generatorcircuit 254 includes a band-stop filter 276 and a phase inverter circuit278. The hand-stop litter 276 attenuates to an extremely low level onlyfrequency signals near a specific frequency signal (for example, 16 MHz)generated by the oscillator 260, The hand-stop filter 276 passes otherfrequency signals (that is, the noise signal) as-is. An output signal282 from the band-stop filter 276 is sent to the phase inverter circuit278, and the phase is inverted by the phase inverter circuit 278.Inverting the phase means advancing the phase 180 degrees.

The noise reduction signal 252 that is the output from the phaseinverter circuit 278 is sent, to the adder circuit 258 and added to theoutput signal 274 in the adder circuit 258. The noise reduction signal252 is a signal that cancels out only the noise component included inthe output signal 274, The noise-reduced signal 256 that is the outputfrom the adder circuit 258 is a signal in which the signals offrequencies other than the specific frequency generated by theoscillator 260 (that is, noise) is reduced. The noise-reduced signal 256is sent to a detector circuit 280, The detector circuit 280 extracts thesignal outputs X and Y, the phase, and the combined impedance Z (=X+iY)described above from the high-frequency signal. These are DC signals.The output from the detector circuit 280 is the output signal 172.

In the present embodiment, because the noise reduction signal 252 isadded to the output signal 274 to generate the noise-reduced signal 256in which the noise included in the output signal 274 is reduced, the S/Nis improved over the technology of the related art. In the case of usingthe eddy current sensor 50 to measure film thickness, the detectionaccuracy of Cu wiring and the like is Improved over the technology ofthe related, art by reducing the influence of noise, and the performanceof the eddy current sensor 50 is improved,

FIG. 17 will, be used to further describe the processing performed inthe band-stop filter 276, the phase inverter circuit 278, and the addercircuit 258. Hereinafter, the case of performing ideal no use cancelingis assumed. The band-stop filter 276 outputs the output signal 282containing only a noise component. To make the explanation dearer, letthe output signal 282 be Y1=A sin(ωt). Here, A is the amplitude of thenoise (in units of millivolts (mv), for example), ω is the angularfrequency of the noise (in units of radians per second (rad/s), forexample), and t is time (seconds (s)).

The output signal 282 is converted to the noise reduction signal 252 bythe phase inverter circuit 278. Letting Y2 be the noise reduction signal252, then Y2=−Y1. Expressed differently, Y2=A sin(ωt+π). On the otherhand, the output signal 274 output by the detecting coil 73 contains anoise component Y3=A sin(ωt) which is the same as the above noisecomponent Y1=A sin(ωt).

The adder circuit 258 adds the noise reduction signal 252 to the outputSignal 274 and outputs the noise-reduced signal 256. Letting Y4 be thenoise component included in the noise-reduced signal 256, then Y4=Y3+Y2,Because Y3 and Y2 have the same amplitude with the phase shifted 180degrees, Y4=0. The noise-reduced signal 256 is a signal that does notinclude a noise component.

In FIG. 16 , the noise reduction signal 252 is generated from the outputof the detecting coil 73 or the dummy coil 74. The noise reductionsignal 252 may also be generated from a source other than the output ofthe detecting coil 73 or the dummy coil 74. For example, the noisereduction signal 252 may also be generated from the output signal 270 ofthe oscillator 260 or from the excitation, signal 250. In FIG. 16 , aplurality of amplifiers and filters are used, but the noise reductionsignal 252 may also be generated from a signal upstream or downstream ofan amplifier or a filter. Note that when generating the noise reductionsignal 252 from the output of the detecting coil 73 or the dummy coil74, it is preferable to generate the noise reduction signal 252 from theoutput of the detecting coil 73. Because the signal that is ultimatelynecessary is the output from the detecting coil 73, it is preferable touse the output of the detecting coil 73 to generate the noise reductionsignal 252 to remove noise from the detecting coil 73.

Returning to FIG. 16 , the detector circuit 280 will be described. FIG.18 is a block diagram illustrating a synchronous detector circuit 280 ofan eddy current sensor. This diagram illustrates an example of a circuitfor measuring the impedance Z seen on the eddy current sensor 50 sidefrom the AC signal source 52 side. In the circuit for measuring theimpedance Z illustrated in the diagram, a resistance component (R), areactance component (X), an amplitude output (Z), and a phase output(tan⁻¹R/X) associated with changes in film thickness can be extracted.

As described above, a signal source 52 supplies an AC signal to the eddycurrent sensor 50 disposed near the semiconductor wafer WH on which themetal film (or conductive film) mf to be detected is formed. The signalsource 52 is an oscillator of fixed frequency containing a quartzoscillator. The signal source 52 supplies a voltage at a fixed frequencyof 2 MHz, 8 MHz, or 16 MHz, for example. The AC voltage formed by thesignal source 52 is supplied to the eddy current, sensor 50 through, abandpass filter 82. A signal detected at the terminal of the eddycurrent, sensor 50 passes through a high-frequency amplifier 83 and aphase shift circuit 84, and the cos component and the sin component ofthe detection signal are extracted by a synchronous detection unitcontaining a cos synchronous detector circuit 85 and a sin synchronousdetector circuit 86. Here, the oscillation signal formed by the signalsource 52 is used by the phase shift circuit 84 to synthesize twosignals of the in-phase component (0°) and the orthogonal component(90°) of the signal source 52, which are introduced into the cossynchronous detector circuit 85 and the sin synchronous detector circuit86 respectively to perform the synchronous detection described above.

In the synchronously detected signals, an unwanted high-frequencycomponent above the signal component is removed by low-pass filters 87and 88 to extract the resistance component (R) output as the cossynchronous detection output and the reactance component (X) output asthe sin synchronous detection output, respectively. Additionally, theamplitude output (R²+X²)^(1/2) is obtained, from the resistancecomponent (R) output and the reactance component (X) output by a vectoroperator circuit 89. Also, the phase output (tan⁻¹R/X) is obtainedsimilarly from the resistance component output and the reactancecomponent output by a vector operator circuit 90. Here, the main body ofthe measuring apparatus is provided with various fitters for removingnoise, components' from sensor signals. A corresponding cutoff frequencyis set for each of the various Oilers, and by setting the cutofffrequency of the Sow-pass filters in the range from 1 Hz to 10 Hz forexample, the noise component mixed with the sensor signal being polishedis removed, and the metal film (or conductive film) targeted formeasurement can be measured accurately.

Next, an exemplary configuration of the coils in the eddy current sensor50 according to the present embodiment will be described. FIG. 5 is aschematic diagram illustrating an exemplary configuration of a coil inthe eddy current sensor 50 of the present embodiment. In the presentembodiment, the eddy current sensor 50 includes an exciting coil 72 forforming an eddy current in the metal film (or conductive film), adetecting coil 73 for defecting the eddy current of the metal film (orconductive film), and a dummy coil 74, The eddy current sensor 50includes the coils of the exciting coil 72, the detecting coil 73, andthe dummy coil 74 in three layers wound around a ferrite core 71. Notethat the structure of the eddy current sensor 50 is not limited to thestructure illustrated in FIG. 5 , and any structure may be adopted.

Here, the exciting coil 72 in the center is connected to the AC signalsource 52. The exciting coil 72 forms an eddy current in the metal film(or conductive film) mf on the semiconductor wafer WH disposed near theeddy current sensor 50 due to a magnetic field formed by the voltagesupplied by the AC signal source 52. The detecting coil 73 is disposedon the upper side (the metal film (or conductive film) side) of theferrite core 71, and detects the magnetic field generated by the eddycurrent formed in the metal film (or conductive film). Additionally, thedummy coil 74 is disposed on the side of the exciting coil 72 oppositethe detecting coil 73. The exciting coil 72 the detecting coil 73, andthe detecting coil 73 are coils having the same number of turns (from 1t to 20 t), for example. The reason for providing the dummy coil 74 isto enable the output from the output signal processing circuit 54 to beadjusted to zero when a metal film (or conductive film) is not present.

There are various possible methods of processing the output from thedetecting coil 73 and the dummy coil 74. For example, as illustrated inFIG. 16 , noise canceling and synchronous detection is performed on theoutput, from each of the detecting coil 73 and the dummy coil 74.Thereafter, the two obtained DC signals are subtracted. The filmthickness can be measured on the basis of the subtraction result. Thereason for subtracting is to cause the output from the detecting coil 73to be zero when a metal film is not present, as described above.

FIGS. 19A, 19B, and 19C are schematic diagrams illustrating anotherexemplary connection of each coil in an eddy current sensor. In thisexample, a resistance bridge Circuit 77 is used. As illustrated in FIG.19A, the detecting coil 73 and the dummy coil 74 are connected inreverse phase with each other. The detecting coil 73 and the dummy coil74 form a reverse-phase series circuit, both ends of which are connectedto a resistance bridge circuit 77 including a variable resistance 76.

Specifically, a signal line 731 of the detecting coil 73 is connected toa terminal 773 of the resistance bridge circuit 77, and a signal line732 of the detecting coil 73 is connected to a terminal 771 of theresistance bridge circuit 77, A signal line 741 of the dummy coil 74 isconnected to a terminal 772 of the resistance bridge circuit 77, and asignal line 742 of the dummy coil 74 is connected to the terminal 771 ofthe resistance bridge circuit 77. The terminal 771 is grounded. Aterminal 774 of the resistance bridge circuit 77 is the sensor output.The detecting coil 73, the exciting coil 72, and the dummy cod 74 haveinductances L₁, L₂, and L₃, respectively.

By connecting the exciting coil 72 to the AC signal source 32 andgenerating alternating flux, an eddy current is formed in the metal,film (or conductive film) mf disposed nearby. By adjusting theresistance value of the variable resistance 76, the output voltage ofthe series circuit containing the detecting coil 73 and the dummy coil74 is adjustable so as to be zero when a metal film (or conductive film)is not present. The L₁ and L₂ signals are adjusted- to be in the samephase with each other by the variable resistance 76 (VR₁ and VR₂)inserted hi parallel with each of the detecting coil 73 and the dummycoil 74. In other words, in the equivalent circuit in FIG. 19B, variableresistances VR₁(=VR₁₋₁+VR₁₋₂) and VR₂(=VR₂₋₁+VR₂₋₂) are adjusted suchthatVR₁₋₁×(VR₂₋₂ +jωL ₃)=VR₁₋₂×(VR₂₋₁ +jωL ₁)  (1)With this arrangement as illustrated in FIG. 19C, the L₁ and L₃ signalsbefore adjustment (indicated by the dashed lines in the diagram) are setto signals having the same phase and the same amplitude (indicated bythe solid line in the diagram).

Next, a different embodiment of the present invention will be described.FIGS. 8 and 9 are schematic diagrams illustrating an exemplaryconfiguration of the eddy current sensor 50 and an exemplary connectionof the exciting coil in the eddy current sensor according to the presentembodiment. The eddy current sensor 50 disposed near the substrate onwhich a conductive film is formed includes a pot core 60 and six coils860, 862, 864, 866, 868, and 870, The pot core 60 is a magnetic materialand has a floor part 61 a (bottom magnetic material), a magnetic corepart 61 b (central magnetic material) provided in the center of thebottom face part 61 a, and a surrounding wall part 61 c (peripheralmagnetic material) provided at the periphery of the floor part 61 a, Thesurrounding wall part 61 c is a wall provided at the periphery of thefloor part 61 a so as to surround the magnetic core part 61 b. In thepresent embodiment, the floor part 61 a has a circular disc shape, themagnetic core part 61 b has a solid columnar shape, and the surroundingwall part 61 c has a cylindrical shape surrounding the floor part 61 a.

Of the six coils 860, 862, 864, 866, 868, and 870, the coils 860 and 862in the center are exciting coils connected to the AC signal, source 52.The exciting coils 860 and 862 form an eddy current in the metal film(or conductive film) mf on the semiconductor wafer WH disposed nearbydue to a magnetic field, formed by the voltage supplied by the AC signalsource 52. The detecting coils 864 and 866 are disposed on the metalfilm side of the exciting coils 860 and 862, and detect the magneticfield generated by the eddy current formed in the metal, film. The dummycoils 868 and 870 are disposed on the opposite side from the detectingcoils 864 and 866 with the exciting coils 860 and 862 in between.

The exciting coil 860 is an Inner coil disposed on the outer peripheryof the magnetic core part 61 b, is capable of generating a magneticfield, and forms an eddy current in a conductive film. The exciting coil862 is an outer toil disposed on the outer periphery of the surroundingwall part 61 e, is capable of generating a magnetic field, and forms aneddy current hi a conductive film. The detecting coil 864 is disposed onthe outer periphery of the magnetic core part 61 b, is capable ofdetecting a magnetic field, and detects an eddy current formed in aconductive film. The detecting coil 866 is disposed on the outerperiphery of the surrounding wall part 61 c, is capable of detecting amagnetic field, and detects an eddy current formed in a conductive film.

The eddy current sensor includes the dummy coils 868 and 870 that detectan eddy current formed in a conductive film. The dummy coil 868 isdisposed on the outer periphery of the magnetic core part 61 b, and iscapable of detecting a magnetic field. The dummy coil 870 is disposed onthe outer periphery of the surrounding wall part 61 c, and is capable ofdetecting a magnetic field. In the present, embodiment, the detectingcoils and the dummy coils are disposed on the outer periphery of thefloor part 61 a and the outer periphery of the surrounding wall part 61c, hot the detecting coils and the dummy coils may al so be disposed ononly one of either the outer periphery of the floor part 61 a or theouter periphery of the surrounding wall part 61 c.

The axial direction of the magnetic core part 61 b is orthogonal to theconductive film on the substrate, the detecting coils 864 and 866, theexciting coils 860 and 862, and the dummy coils 868 and 870 are disposedat different positions m the axial direction of the magnetic core part61 b, and additionally, the detecting coils 864 and 866, the excitingcoils 860 and 862, and the dummy coils 868 and 870 are disposed in theabove order in the axial direction of the magnetic core part 61 bproceeding from a position close to the conductive film on the substrateto a position farther away. A lead line (illustrated in FIG. 20 ) forconnecting to the outside extends from each of the detecting coils 864and 866, the exciting coils 860 and 862, and the dummy coils 868 and870.

FIG. 8 is a cross-section of the eddy current sensor 50 in a planepassing through a central axis 872 of the magnetic core part 61 b. Thepot core 60 is a magnetic material and has a columnar floor part 61 a, acolumnar magnetic core part 61 b provided in the center of the floorpart 61 a, and a cylindrical surrounding watt part 61 c provided on thecircumference of the floor part 61 a. As one example of the dimensionsof the pot core 60, the diameter LE1 of the floor part 61 a isapproximately from 1 cm to 5 cm, and the height LE2 of the eddy currentsensor 50 is approximately from 1 cm to 5 cm. The outer diameter of thesurrounding wall part 61 c has the same cylindrical shape in the heightdirection in FIG. 8 , but the outer diameter may also have a convergingshape (tapered shape) that narrows in the direction going away from thefloor part 61 a, or in other words, narrows toward the tip.

The conducting wire used in the detecting coils 864 and 866, theexciting coils 860 and 862, and the dummy coils 868 and 870 is copper,Manganin wire, or nichrome wire. As a result of using Manganin wire ornichrome wire, there are fewer temperature changes due to electricalresistance and the like, and the temperature properties are improved.

In the present embodiment, wire rod is wrapped around the outside of themagnetic core part 61 b and the outside of the surrounding wall part 61c made of ferrite or the like to form the exciting coils 860 and 862,and therefore the eddy current density flowing through the measurementtarget can be raised. Additionally, because the detecting coils 864 and866 are also formed, on the outside of the magnetic core part 61 b andthe outside of the surrounding wall part 61 c, the generated reversemagnetic field (interlinkage flux) can be collective efficiently.

To raise the eddy current density flowing through the measurementtarget, in the present embodiment, the exciting coil 860 and theexciting coil 862 are additionally connected, in parallel, asillustrated in FIG. 9 . In other words, the inner coil and the outercoil are electrically connected in parallel. The reasons for connectingin parallel are as follows. When connected in parallel, the voltage thatcan be applied to the exciting coil 860 and the exciting coil 862 isincreased and the current flowing through the exciting coil 860 and theexciting coil 862 is increased over the case of connecting in series.Consequently, the magnetic field is larger. Also, when connected inseries, the inductance of the circuit increases, and the frequency ofthe circuit falls. This makes it difficult to apply the necessary highfrequency to the exciting coils 860 and 862, The arrow 874 indicates thedirection of current flowing through the exciting coil 860 and theexciting coil 862.

As illustrated in FIG. 9 , the exciting coil 860 and the exciting coil862 are connected such that the exciting coil 860 and the exciting coil862 have the same magnetic field direction. In other words, currentflows in different directions in the exciting coil 860 and the excitingcoil 862. A magnetic field 876 is the magnetic field generated by theexciting coil 860 on the inner side, while a magnetic field 878 is themagnetic field generated by the exciting coil. 862 on the outer side. Asillustrated in FIG. 9 , the magnetic field directions of the excitingcoil 860 and the exciting coil 862 are the same. In other words, thedirection of the magnetic field that the inner coil generates inside themagnetic core part. 61 b is the same as the direction of the magneticfield that the outer coil generates inside the magnetic core part 61 b.

Because the magnetic field 876 and the magnetic field 878 illustrated inthe region 880 have the same orientation, the two magnetic fields areadded together and become larger. Compared to the case, where only themagnetic field 876 generated by the exciting coil 860 exists like in therelated art, in the present embodiment, the magnetic field is larger bythe extent of the magnetic field 878 generated by the exciting coil 862.

The detecting coil 864, the exciting coil 860, and the dummy cod 868correspond to the detecting coil 73, the exciting coil 72, and the dummycoil 74 of FIG. 5 . The detecting coil 866, the exciting coil 862, andthe dummy coil 870 correspond to the detecting coil 73, the excitingcoil 72, and the dummy coil 74 of FIG. 5 . In other words, the eddycurrent sensor of FIG. 10 has a structure in which two of the eddycurrent sensor in FIG. 5 are disposed concentrically. Accordingly, theoutput signal processing circuit 54 corresponding to the eddy currentsensor of FIG. 10 preferably includes two of the output signalprocessing circuit 54 illustrated hi FIG. 16 .

An example of the output signal processing circuit 54 corresponding tothe eddy-current sensor of FIG. 10 is illustrated in FIGS. 20 and 21 ,FIG. 20 is a comparative example that does not include a noise cancelingfunction. FIG. 21 includes a noise canceling function. FIGS. 20 and 21illustrate a perspective view of the eddy current sensor illustrated inFIG. 8 , In FIGS. 20 and 21 , a top face 218 is illustrated above a topface 220 for easier understanding, but the top lace 218 and the top lace220 are in the same horizontal plane as illustrated in FIG. 8 . In FIGS.20 and 21 , there are two coil assemblies, but there may also be threeor more coil assemblies. In the case where there are two or more coilassemblies, the measurement accuracy (S/N ratio) is improved because ofthe increased number of film thickness measurements compared to the caseof a single coil assembly.

First, the configuration of the comparative example will be described.In the comparative example, a detecting coil 864 of an inner eddycurrent sensor 286 and a detecting coil 866 of an outer eddy currentsensor 288 are connected in series. A dummy coil 868 of the tuner eddycurrent sensor 286 and a dummy coil 870 of the outer eddy current sensor288 are connected in series. An exciting coil 860 and an exciting coil862 of the inner and outer eddy current sensors are connected inparallel to the signal source 52.

The specific connections are as follows. In the inner eddy currentsensor 286, the detecting coil 864 includes signal lines 8641 and 8642.The exciting coil. 860 includes signal Hues 8601 and 8602, The dummycoil 868 includes signal lines 8681 and 8682. In the outer eddy currentsensor 288, the detecting coil 866 includes signal lines 8661 and 8662,The exciting coil 862 includes signal, lines 8621 and 8622. The dummycoil 870 includes signal lines 8701 and 8702.

The signal line 8641 of the detecting coil 864 of the inner eddy currentsensor 286 is connected to the terminal 773 of the resistance bridgecircuit 77. The signal line 8642 of the detecting coil 864 is connectedto the signal line 8661 of the detecting coil 866 of the outer eddycurrent sensor 288. The signal line 8662 of the detecting coil 866 isconnected to the terminal 771 of the resistance bridge circuit 77. Thesignal line 8681 of the dummy coil. 868 of the inner eddy current sensor286 is connected to the terminal 772 of the resistance bridge circuit77. The signal line 8642 of the dummy coil 868 is connected to thesignal line 8701 of the dummy coil 870 of the outer eddy current sensor288. The signal line 8702 of the dummy coil 870 is connected to theterminal 771 of the resistance bridge circuit 77.

In the comparative example, the output of detecting coil 864 of theinner eddy current sensor 286 and the output of the detecting coil 866of the outer eddy current sensor 288 are in series, and consequentlythere is an effect of increased output compared to the case of a singledetecting coil. The terminal 774 of the resistance bridge circuit 77 isconnected to the detector circuit 280. The output of the detectorcircuit 280 is the output signal 172, and is connected to the endpointdetection controller 246 illustrated in FIG. 15 .

The embodiment illustrated in FIG. 21 includes a noise cancelingfunction, and therefore has the advantage of an improved S/N over thecomparative example. The present embodiment uses a separate resistancebridge circuit 77 for each of the inner eddy current sensor 286 and theouter eddy current sensor 288. Additionally, because a separate outputsignal processing circuit 54 (generator circuit 254 and adder circuit258) is provided downstream of each resistance bridge circuit 77, theS/N is improved.

The generator circuit 254 for the inner eddy current sensor 286 is agenerator circuit 2541, while the generator circuit 254 for the outereddy current sensor 288 is a generator circuit 2542, The generatorcircuit 2541 and the generator circuit 2542 have the same configurationas the generator circuit 254. The filter 266 and the amplifier 268 areprovided upstream of the generator circuit 2541 and the generatorcircuit 2542, similarly to FIG. 16 . The adder circuit 258 for the innereddy current sensor 286 is an adder circuit 2581, while the addercircuit 258 for the outer eddy current sensor 288 is an adder circuit2582. The adder circuit 2581 and the adder circuit 2582 have the sameconfiguration as the adder circuit 258.

The noise-reduced signals 256 obtained by the inner eddy current sensor286 and the outer eddy current sensor 288 are sent to the detectorcircuit 280, and the outputs of the two bridge circuits 7710 and 7711are in series. Thereafter, the signals are added together in an addercircuit 284. Adding the signals together improves the sensitivity.

The configuration of the present embodiment will be describedspecifically. The inner eddy current sensor 286 includes the excitingcoil 860 (First exciting coil) that accepts excitation signals 8601 and8602 from the signal source 52 as input and is capable of generating thefirst magnetic field 876 (see FIG. 10 ), the detecting coil 864 (firstdetecting coil) capable of detecting the first magnetic field 876 andoutputting a signal 8641 (first detection signal), and the dummy coil868 (first dummy coil) capable of detecting the first magnetic field andoutputting a signal 8681 (first dummy signal). Note that in thisspecification, a signal line and a signal that flows on that signal linemay be denoted with the same reference sign in some cases, such as thesignal line 8641 and the signal 8641, for example.

An output signal processing circuit 290 of the present embodiment thatprocesses the signal 8641 and the signal 8681 output by the inner eddycurrent sensor 286 includes a first resistance bridge circuit 7710capable of outputting the difference between the signal 8641 and thesignal 8681 as a first difference signal 292. The output signalprocessing circuit 290 includes the generator circuit 2541 (firstgenerator circuit) capable of generating a first noise reduction signal252 for reducing noise from any of the excitation signal 8601, thesignal 8641 (first detection signal), the signal 8681 (first dummysignal), and the first difference signal 292. In the present,embodiment, the first noise reduction signal 252 for reducing noise isgenerated from the first difference signal 292 that is the output fromthe terminal 774.

The output signal processing circuit 290 further includes a first addercircuit 2581 capable of adding the first noise reduction signal 252generated in the generator circuit 2541 to the first difference signal.292, and thereby generating a first noise-reduced signal 256 in whichthe noise included in the first difference signal 292 is reduced.

The outer eddy current sensor 288 includes the exciting coil 862 (secondexciting coil) that accepts an excitation signal 8621 as input and iscapable of generating the second magnetic field 878 (see FIG. 10 ), thedetecting coil 866 (second detecting coil) capable of detecting thefirst magnetic field 876 and the second magnetic field 878 andoutputting a signal 8661 (second detection signal), and the dummy coil870 (second dummy coil) capable of detecting the first magnetic field876 and the second magnetic field 878 and outputting a signal 8701(second dummy signal).

The detecting coil 864 of the inner eddy current sensor 286 is capableof detecting the first magnetic field 876 and the second magnetic field878 and outputting the signal 8641. The dummy cod 868 is capable ofdetecting the first magnetic field 876 and the second magnetic field 878and outputting the first dummy signal. The output signal processingcircuit 290 includes a second resistance bridge circuit 7711 capable ofoutputting the difference between the signal 8661 and the signal 8701 asa second difference signal 294.

Here, the meaning of the first and second detecting coils detecting thefirst magnetic field 876 and the second magnetic field 878 will bedescribed. The detecting coil 864 of the inner eddy current sensor 286detects the film thickness as follows. First, an eddy current is inducedon the metal surface by the high-frequency magnetic Held generated bythe exciting coil. In other words, the state of the magnetic field isdifferent depending on whether the wafer (conductive film) is nearby ornot. An exciting coil capable of generating a magnetic field on themetal surface by applying a high frequency (2 MHz or higher) to theexciting coil is brought close to the conductive metal surface. Thehigh-frequency magnetic field generates current in eddies on the metalsurface. The eddy current flows in a direction that partially cancelsout the magnetic field. The detecting coil of the eddy current sensordetects the magnetic field that is partially canceled out. The eddycurrent sensor uses the correlation between the magnitude of the eddycurrent and the thickness of the film to measure the thickness of theconductive film, on the metal.

“Detecting the first magnetic field and the second magnetic field”refers to “defecting the combined magnetic field of the first magneticfield and the second magnetic field”, and the combined magnetic field isalso different depending on whether the wafer (conductive film) ispresent or not, “The first and second detecting coils detect the firstand second magnetic fields” means that “an eddy current is generated onthe metal surface by the combined magnetic field of the first and secondmagnetic fields, the combined magnetic field is partially canceled outby the generated eddy current, and the partially canceled, out magneticfield is detected”.

Returning to the description of the outer eddy current sensor 288, theoutput signal processing circuit 290 includes the generator circuit 2542(second generator circuit) capable of generating a second noisereduction signal 252 for reducing noise from any of the excitationsignal 8621, the signal 8661 (second detection signal), the signal 8701(second dummy signal), and the second difference signal 294. In thepresent embodiment, the second noise reduction signal 252 for reducingnoise is generated from the second difference signal 294 that is theoutput from the terminal 772.

The output signal processing circuit 290 includes a second adder circuit2582 capable of adding the second noise reduction signal 252 generatedin the generator circuit 2542 to the second difference signal 294 andthereby generating s second noise-reduced signal 256 in which the noiseincluded in the second difference signal 294 is reduced, and a thirdadder circuit 284 capable of adding together the first noise-reducedsignal 256 and the second noise-reduced signal 256.

According to the present embodiment, because the first difference signaland the second difference signal are added together, the output signalfrom the eddy current sensor is larger than the related art and theaccuracy of Him thickness measurement is improved.

A comparison of the effects of the embodiment in FIG. 21 to the effectsof an embodiment having a single eddy current sensor and lacking a noisecanceling function is illustrated conceptually in FIGS. 22 and 23 . Eachof FIGS. 22 and 23 is a spectrum illustrating an intensity distributionof noise and signal contained in an output signal from a detecting coilwith respect to frequency. FIG. 22 illustrates the spectrum of theoutput 300 of an eddy current sensor in an embodiment (not illustrated)having a single eddy current sensor and lacking a noise cancelingfunction. FIG. 23 is the spectrum of the output 302 obtained by addingtogether the outputs from two eddy current sensors after a noisecanceling process in the embodiment illustrated in FIG. 21 that has twoeddy current sensors and also includes a noise canceling function. InFIGS. 22 and 23 , the horizontal axis is frequency (in units offrequency (1/s)), and the vertical axis is the power density (square ofthe amplitude) at each frequency (in units of millivolts squared). Theoutputs 300 and 302 contain a signal 304 that exists in the vicinity ofthe frequency 296, and noise 306 that exists outside the vicinity of thefrequency 296.

Comparing FIGS. 22 and 23 demonstrates that the magnitude of the output302 obtained by adding together the signals 304 from the two eddycurrent sensors in the vicinity of the frequency 296 included in theexcitation signal 8603 is on the order of double the magnitude of thesignal 304 from the single eddy current sensor. The magnitude of thenoise 306 included in the output 302 from the two eddy current sensorsoutside the vicinity of the frequency 296 included in the excitationsignal 8601 is strongly reduced compared to the magnitude of the noise306 included in the output 300 from the single eddy current sensor.

Note that as illustrated in FIG. 21 , the first noise-reduced signal 256and the second noise-reduced signal 256 are added together by the thirdadder circuit 284 after passing through respective detector circuits 280and being converted to DC signals (a first noise-reduced signal 2801 anda second noise-reduced signal 2802). Comparing FIGS. 20 and 21 demonstates that the embodiment of FIG. 21 is the embodiment of FIG. 20 withthe addition of the first resistance, bridge circuit 7710, the firstgenerator circuit 2541, the second generator circuit 2542, the firstadder circuit 2581, the second adder circuit 2582, the detector circuit280 for the inner eddy current sensor 286, and the third adder circuit284.

From the embodiment of FIG. 21 , an embodiment without a noise cancelingcircuit is also possible. In other words, the outputs of the two eddycurrent sensors 286 and 288 are respectively processed, by the bridgecircuits 7710 and 7711, respectively converted to DC signals by thedetector circuits 280 for respectively processed signals, and then,added together by the third adder circuit 284.

Specifically, the output signal processing circuit in this embodimentincludes the first resistance bridge circuit. 7710 capable of outputtingthe difference between the signal 8641 (first detection signal) and thesignal. 8681 (first dummy signal) as a first difference signal 292, thesecond resistance bridge circuit 7711 capable of outputting thedifference between the signal. 8661 (second detection signal) and thesignal 8701 (second dummy signal) as a second difference signal 294, andthe third adder circuit 284 capable of adding together the firstdifference signal 292 and the second difference signal 294 via thedetector circuits 280.

In FIG. 21 , the signals are added together by the third adder circuit284 after DC conversion by the detector circuits 280, but the DCconversion by the detector circuits 280 does not have to be performed,in other words, AC signals may be added together. However, addingtogether the signals after performing DC conversion has the advantagesdescribed below.

Compared to FIG. 20 , FIG. 21 has the following advantages. In FIG. 20 ,the outputs of the detecting coil 864 and the detecting coil 866 areadded together by directly coupling the signal line 8642 and the signalline 8661. These are high-frequency signals, and high-frequency signalsmay cancel each other out when added together. As a result, the outputsignals may not be utilized effectively in some cases, in FIG. 21 , thesignals are added together by the third adder circuit 284 afterperforming DC conversion by the detector circuits 280, Because DCsignals are added together, the signals do not cancel each other out,and the output signal level increases.

In the embodiment of FIG. 21 , the third adder circuit 284 adds togetherthe output of the inner eddy current sensor 286 and the output of theouter eddy current sensor 288, but the third adder circuit 284 may alsosubtract the output of the outer eddy current sensor 288 from the outputof the inner eddy current sensor 286. This case has the followingadvantages. The inner eddy current sensor 286 has a small diameter andtherefore is susceptible to the influence of a conductive film in asmall region. As a result, the film thickness in a small region can bemeasured. The outer eddy current sensor 288 has a large diameter andtherefore is susceptible to the Influence of a conductive film in alarge region. As a result, the film thickness in a large region can bemeasured. If the output of the outer eddy current sensor 288 issubtracted from the output, of the inner eddy current sensor 286, theinfluence of a conductive film in a large region can be removed from theoutput of the inner eddy current sensor 286. As a result, the filmthickness in a small region can be measured more accurately. The term“adding together” in the third adder circuit 284 illustrated in FIG. 21encompasses “addition” and “subtraction” in this sense.

Note that in the eddy current sensor illustrated in FIG. 8 , the innereddy current sensor 286 and the outer eddy current sensor 288 areintegrated, but the inner eddy current sensor 286 and the outer eddycurrent sensor 288 may also be separate and independent eddy currentsensors. Methods of producing the integrated eddy current sensorillustrated in FIG. 8 include (1) a method of forming the floor part 61a, the magnetic core part 61 b, and the surrounding wall part 61 c as asingle body by cutting, (2) a method of producing the floor part 61 a,the magnetic core part 61 b, and the surrounding wall part 61 cseparately, and after winding the coils, joining and integrating thethree pieces by brazing or the like.

The band-stop filter 276, the phase inverter circuit 278, and the addercircuit 258 can be achieved by a digital signal processor (DSP). Adigital signal processor is a microprocessor suitable for digital signalprocessing. The band-stop filter 276, the phase inverter circuit 278,and the adder circuit 258 may also be analog circuits. The digitalsignal processor includes an upstream analog-to-digital convertercircuit (that is, an electronic circuit that converts an analogelectrical signal to a digital electrical signal; also referred to as anA/D converter circuit or an ADC) not illustrated. The digital signalprocessor includes a downstream digital-to-analog converter circuit(that is, an electronic circuit that converts a digital electricalsignal to an analog electrical signal; also referred to as a D/Aconverter circuit or a DAC) not illustrated. The detector circuits 280downstream of the adder circuit 258 can also be achieved by a digitalsignal processor.

Note that in the description of FIG. 17 , the noise component Y1 and thenoise component Y3 are assumed to be the same in the magnitude of theamplitude A to simplify the description, but the noise component. Y1 andthe noise component Y3 may be different from the magnitude of theamplitude A in some cases. For example, in some cases, the noisecomponent Y1 is generated from the excitation signal 250 while the noisecomponent Y3 is the noise component included in the detecting coil 73.At this time, a process of matching the magnitude of the amplitude A isperformed in the DSP. In the description of FIG. 17 , the noisecomponent Y1 and the noise component Y3 are assumed to have the samephase to simplify the description, bat in the case of a large phasemisalignment, a process of aligning the phase is performed in the DSP.

Note that in the adder circuit 258 of FIG. 16 , the output signal 274 isadded together with a signal of inverted phase relative to the outputsignal 274, The method of noise canceling is not limited to the above,and any method capable of noise canceling can be adopted. For example,there is a method of subtracting signals without inverting the phase. Inother words, the phase inverter circuit 278 is removed, and the outputsignal of the hand-stop filter 276 is subtracted from the output signal274 in the adder circuit 258. With this method, noise canceling similarto the case of using the phase Inverter circuit 278 is possible.

Consequently, “adding a signal of inverted phase” and “subtracting asignal of non-inverted phase” have the same effect from the viewpoint ofnoise canceling. In this specification, the term “adding together”encompasses “addition” and “subtraction” in this sense.

Note that by disposing a temperature sensor inside the eddy currentsensor 50, temperature changes Inside the eddy current sensor 50 can bedetected, and the output of the detecting coil 73 cart be corrected. Thecorrection method is as follows, for example. Before polishing thesemiconductor wafer WH, the relationship between the temperature of theeddy current sensor 50 and the output of the detecting coil 73 ismeasured, and the relationship is created as a correction table. Thecorrection table may have any of various possible configurations. Forexample, a coefficient by which to multiply the output of the detectingcoil 73 for each temperature of the eddy current sensor 50 is created asfee correction table. Alternatively, a polynomial expressing correctioncoefficients as a function of temperature is created as the correctiontable (a table displaying the coefficient of each order in thepolynomial). Correcting according to temperature makes it possible toaccommodate temperature changes. The film thickness measurement accuracycan be raised, and the accuracy of detecting the end of polishing can beimproved.

Next, a method of processing the output signal from the eddy currentsensor will be described. In FIG. 16 , the exciting coil 72 accepts theexcitation signal 250 as input and generates a magnetic field. Thedetecting coil 73 detects the magnetic field and outputs the outputsignal 176. The generator circuit 254 generates the noise reductionsignal 252 for reducing noise from the excitation signal 250 or theoutput signal 176, The adder circuit 258 adds the generated noisereduction signal 252 to the output signal 176 to generate thenoise-reduced signal 256 in which the noise included in the outputsignal 176 is reduced.

Next, a different method of processing the output signal from the eddycurrent sensor will be described. In FIG. 21 , the exciting coil 860(first exciting coil) accepts the excitation signal 250 as input andgenerates the first magnetic field 876, The detecting coil 864 (firstdetecting coil) detects the first magnetic field 876 (see FIG. 10 ) andoutputs the signal 8641 (first detection signal). The dummy coil 868(first dummy coil) detects the first magnetic field 876 and outputs thesignal 8681 (first dummy signal).

In the eddy current sensor output signal processing method thatprocesses the signal 8641 and the signal 8681 output by the eddy currentsensor 286, the first resistance bridge circuit 7710 outputs thedifference between the signal 8641 and the signal 8681 as the firstdifference signal 292. The generator circuit 2541 generates the firstnoise reduction signal 252 for reducing noise from any of the excitationsignal 250, the signal 8641, the signal 8681, and the first differencesignal 292, In FIG. 21 , the generator circuit 2541 generates the firstnoise reduction signal 252 for reducing noise from the first differencesignal 292. The first adder circuit 2581 adds the generated first noisereduction signal 252 to the first difference signal 292 to generate thefirst noise-reduced signal 2801 in which the noise included in the firstdifference signal 292 is reduced.

Next, a different method of processing the output signal from the eddycurrent sensor will be described. In FIG. 21 , the exciting coil. 862(second exciting cod) accepts the excitation signal 250 as input, andgenerates the second, magnetic field 878 (see FIG. 10 ). The detectingcoil 866 (second detecting coil) detects the first magnetic field 876and the second magnetic field 878, and outputs the signal 8661 (seconddetection signal). The dummy coil 870 (second dummy coil) detects thefirst magnetic field 876 and the second magnetic field 878, and outputsthe signal 8701 (second dummy signal).

The detecting coil 864 detects the first magnetic field 876 and thesecond magnetic field 878, and outputs the signal 8641 (first detectionsignal). The dummy coil 868 detects the first magnetic field 876 and thesecond magnetic field 878, and outputs the signal 8681 (first dummysignal). The second resistance bridge circuit 7711 outputs thedifference between the signal 8661 and the signal 8701 as the second,difference signal 294. The generator circuit 2542 generates the secondnoise reduction signal 252 for reducing noise from any of the excitationsignal 250, the signal 8661, the signal 8701, and the second differenceSignal 294, The second adder circuit 2582 adds the generated secondnoise reduction signal 252 to the second difference signal 294 togenerate the second noise-reduced signal 2802 in which the noiseincluded in the second difference signal 294 is reduced. The third addercircuit 284 adds together the first noise-reduced signal 281 and thesecond noise-reduced signal 2802.

Next, a different method of processing the output signal from the eddycurrent sensor will be described. The present embodiment is obtained byremoving the noise canceling, circuit from the embodiment in FIG. 21 .The outputs from the two eddy current sensors 286 and 288 are processedby the bridge circuits 7710 and 7711, respectively. Thereafter, thesignals are converted to DC signals by the detector circuit 280 and thenadded together in the third adder circuit 284. Specifically, theexciting coils 860 and 862 accept the excitation, signal 250 as inputand generate the first magnetic field 876 and the second magnetic field878, respectively. The detecting coils 864 and 866 (first and seconddetecting coils) detect the first magnetic field 876 and the secondmagnetic field 878, and output the signals 8641 and 8661 (first andsecond detection signals), respectively. The dummy coils 868 and 870(first and second dummy coils) detect the first magnetic field 876 andthe second magnetic field 878, and output the signals 8681 and 8701(first and second dummy signals), respectively.

In the output, signal processing method of the present embodiment, thesignals 8641 and 8661 and the signals 8681 and 8701 output by the eddycurrent sensors 286 and 288 are processed. The first resistance bridgecircuit 7710 outputs the difference between the signal 8641 (firstdetection signal) and the signal 8681 (first dummy signal) as the firstdifference signal 292. The second resistance bridge circuit 7711 outputsthe difference between the signal 8661 (second detection signal) and thesignal 8701 (second dummy signal) as the second difference signal 294.The first difference signal 292 and the second difference signal 294 areconverted to DC signals in the detector circuit 280 and then addedtogether in the third adder circuit 284.

A method of controlling each unit of the polishing apparatus on thebasis of the film thickness obtained by the eddy current sensor 50 isdescribed hereinafter. As illustrated in FIG. 15 , the eddy currentsensor 50 is connected to an endpoint, detection controller 246, and theendpoint detection controller 246 is connected to an equipmentcontroller 248. The output signal from the eddy current sensor 50 issent to the endpoint detection controller 246. The endpoint detectioncontroller 246 performs necessary processing (arithmetic processing andcorrection) on the output signal from the eddy current sensor 50 togenerate a monitoring signal (film thickness data corrected by theendpoint detection controller 246), The equipment controller 248controls components such as the top ring motor 114 and a motor for thepolishing table 100 (not illustrated) on the basis of the corrected filmthickness data.

Note that the operations according to the embodiments of the presentinvention are also achievable using the following software and/orsystem. For example, the system (polishing apparatus) includes a maincontroller (control unit) that controls the apparatus overall and aplurality of sub-controllers that respectively control the operations byeach unit (driving unit, holding unit, eddy current sensor output signalprocessing circuit, endpoint detection controller). The main controllerand the sub-controllers each include a CPU, memory; a recording medium,and software (a program) stored in the recording medium for causing eachunit to operate. The method for processing an output signal from an eddycurrent sensor according to an embodiment of the present invention mayalso be executed by the software (program).

The foregoing describes exemplary embodiments of the present invention,but the embodiments described above are for facilitating theunderstanding of the present invention, and do not limit the presentinvention. The present invention may be modified and improved withoutdeparting from the scope of the invention, and any equivalents obtainedthrough such modification and improvement are obviously included in thepresent invention. Furthermore, any combination, or omission of thecomponents described in the claims and the specification is possibleinsofar as at least one or some of the issues described above can beaddressed, or insofar as at least one or some of the effects areexhibited.

This application claims the benefit of priority under the ParisConvention for Japanese Patent Application No. 2019-226547 and JapanesePatent Application No. 2019-226549 filed on Dec. 16, 2019 at the JapanPatent Office. The entire disclosure of Japanese Patent Laid-Open No.2011-23579, including specification, claims, drawings, and abstract, isincorporated by reference as a part of this specification.

REFERENCE SIGNS LIST

-   -   50 eddy current sensor    -   54, 290 output signal processing circuit    -   72, 860, 862 exciting coil    -   73, 864, 866 detecting coil    -   74, 868, 870 dummy coil    -   174 eddy current sensor assembly    -   178, 1781, 1.782 mixer circuit    -   184, 1841, 1842 low-pass fitter    -   216 digital signal processor    -   246, 248 endpoint detection, controller    -   252 noise reduction signal, first noise reduction signal, second        noise reduction signal    -   256 noise-reduced signal, first noise-reduced signal, second        noise-reduced signal    -   258 adder circuit    -   284 adder circuit, third adder circuit    -   292 first difference signal    -   294 second difference signal    -   1783 third mixer circuit    -   1784 fourth mixer circuit    -   1843 third low-pass filler    -   1844 fourth low-pass filter    -   1941 first adjustment circuit    -   1942 second adjustment circuit    -   1961 first subtracter circuit    -   2541, 2542 generator circuit    -   2581 first adder circuit    -   2582 second adder circuit    -   2801 first noise-reduced signal    -   2802 second noise-reduced signal

What is claimed is:
 1. An eddy current sensor output signal processingcircuit, that processes first and second output signals output by aneddy current sensor including first and second coils that respectivelyoutput the first and second output signals, the output signal processingcircuit comprising: a first mixer circuit that accepts the first outputsignal and a signal of a predetermined frequency as input, multipliesthe two input signals, and outputs an output signal obtained by themultiplication; a first low-pass filter that accepts the output signaloutput by the first mixer circuit as input, cuts a high-frequency signalincluded in the output signal received as input, and outputs at least aDC signal; a second mixer circuit that accepts the second output signaland the signal of the predetermined frequency as input, multiplies thetwo input signals, and outputs an output signal obtained by themultiplication; a second low-pass filter that accepts the output signaloutput by the second mixer circuit as input, cuts a high-frequencysignal included in the output signal received as input, and outputs atleast a DC signal; and a first subtractor circuit that accepts the DCsignal output by the first low-pass filter and the DC signal output bythe second low-pass filter as input, obtains a difference between thetwo input DC signals, and outputs the obtained difference, wherein theeddy current sensor includes third and fourth coils that respectivelyoutput third and fourth output signals, and the output signal processingcircuit includes a third mixer circuit that accepts the third outputsignal and the signal of the predetermined frequency as input,multiplies the two input signals, and outputs an output signal obtainedby the multiplication, a third low-pass filter that accepts the outputsignal output by the third mixer circuit as input, cuts a high-frequencysignal included in the output signal received as input, and outputs atleast a DC signal, a fourth mixer circuit that accepts the fourth outputsignal and the signal of the predetermined frequency as input,multiplies the two input signals, and outputs an output signal obtainedby the multiplication, a fourth low-pass filter that accepts the outputsignal output by the fourth mixer circuit as input, cuts ahigh-frequency signal included in the output signal received as input,and outputs at least a DC signal, a second subtractor circuit thataccepts the DC signal output by the third low-pass filter and the DCsignal output by the fourth low-pass filter as input, obtains adifference between the two input DC signals, and outputs the obtaineddifference, and an adder circuit that accepts the difference output bythe first subtractor circuit and the difference output by the secondsubtractor circuit as input, obtains a sum of the two input differencesor a difference between the two input differences, and outputs theobtained sum or difference.
 2. The eddy current sensor output signalprocessing circuit according to claim 1, wherein the output signalprocessing circuit includes a first adjustment circuit that accepts theDC signal output by the first low-pass filter as input, adjusts amagnitude of an amplitude of the input DC signal, and outputs anadjusted DC signal, and the first subtractor circuit accepts the DCsignal output by the first adjustment circuit and the DC signal outputby the second low-pass filter as input, obtains a difference between thetwo input DC signals, and outputs the obtained difference.
 3. The eddycurrent sensor output signal processing circuit according to claim 1,wherein the output signal processing circuit includes a first adjustmentcircuit that accepts the DC signal output by the first low-pass filteras input, adjusts a magnitude of an amplitude of the input DC signal,and outputs an adjusted DC signal, and a second adjustment circuit thataccepts the DC signal output by the second low-pass filter as input,adjusts a magnitude of an amplitude of the input DC signal, and outputsan adjusted DC signal, and the first subtractor circuit accepts the DCsignal output by the first adjustment circuit and the DC signal outputby the second adjustment circuit as input, obtains a difference betweenthe two input DC signals, and outputs the obtained difference.
 4. Apolishing apparatus comprising: a polishing table to which a polishingpad for polishing a substrate is attached; a driving unit configured torotationally drive the polishing table; a holding unit configured tohold the substrate and press the substrate against the polishing pad; aneddy current sensor disposed inside the polishing table and configuredto detect an eddy current formed in a conductive film formed on thesubstrate in association with the rotation of the polishing table; aneddy current sensor output signal processing circuit that processesfirst and second output signals output by the eddy current sensorincluding first and second coils that respectively output the first andsecond output signals, the output signal processing circuit comprising:a first mixer circuit that accepts the first output signal and a signalof a predetermined frequency as input, multiplies the two input signals,and outputs an output signal obtained by the multiplication; a firstlow-pass filter that accepts the output signal output by the first mixercircuit as input, cuts a high-frequency signal included in the outputsignal received as input, and outputs at least a DC signal; a secondmixer circuit that accepts the second output signal and the signal ofthe predetermined frequency as input, multiplies the two input signals,and outputs an output signal obtained by the multiplication; a secondlow-pass filter that accepts the output signal output by the secondmixer circuit as input, cuts a high-frequency signal included in theoutput signal received as input, and outputs at least a DC signal; and afirst subtractor circuit that accepts the DC signal output by the firstlow-pass filter and the DC signal output by the second low-pass filteras input, obtains a difference between the two input DC signals, andoutputs the obtained difference; and an endpoint detection controllerconfigured to compute film thickness data about the substrate from theoutput of the output signal processing circuit.
 5. The polishingapparatus according to claim 4, wherein the output signal processingcircuit includes a first adjustment circuit that accepts the DC signaloutput by the first low-pass filter as input, adjusts a magnitude of anamplitude of the input DC signal, and outputs an adjusted DC signal, andthe first subtractor circuit accepts the DC signal output by the firstadjustment circuit and the DC signal output by the second low-passfilter as input, obtains a difference between the two input DC signals,and outputs the obtained difference.
 6. The polishing apparatusaccording to claim 4, wherein the output signal processing circuitincludes a first adjustment circuit that accepts the DC signal output bythe first low-pass filter as input, adjusts a magnitude of an amplitudeof the input DC signal, and outputs an adjusted DC signal, and a secondadjustment circuit that accepts the DC signal output by the secondlow-pass filter as input, adjusts a magnitude of an amplitude of theinput DC signal, and outputs an adjusted DC signal, and the firstsubtractor circuit accepts the DC signal output by the first adjustmentcircuit and the DC signal output by the second adjustment circuit asinput, obtains a difference between the two input DC signals, andoutputs the obtained difference.